1 - the only instruments that will work outside of Sonar are Dim Pro & Rapture
2 - you can buy a small, lightweight, USB powered interface that will solve all of your problems for about $100

Graeme McTaggart
I deleted the project because, once Sonar starts displaying the "master assigned to silent hardware" message, there is no way that I can work out, of restoring that project to the state of being audible.
Every time this happens, if I start a new project during the same session, it will be fine, but the file with the problem stays locked.
The same message will be displayed if I open the project; so, even though Sonar is configured correctly at that moment, it will not reroute that project to the selected, functioning audio device.
It makes no difference whether I'm using speakers or headphones, once I get that message, it's the kiss of death for that project.
 
I am working my way through the 2188 page pdf, learning as I got.
Sonar is basically similar to FL, it is the more crowded interface which is taking time to get used to.
Yesterday's track had a single instance of Strum Session with 12 bars in the piano roll. Nothing else.
I was just starting to programme the track and I'm still getting used to the piano roll in Sonar. (I find it tricky to work with at present).
 
I work abroad, so I never considered buying an audio interface, mainly because I never record anything, and also because it would be additional weight in my luggage and FL works fine on my notebook without one.
I don't know how heavy these things are or their dimensions, but I would either have to abandon it next summer or else pay to have it couriered to my home address.
If the only solution to this problem is to buy one, then in all honesty, I would have to seriously consider paying another £200/$300 to update FL Studio and see how many instruments from Sonar will work with FL.
 
I have gone through the audio hardware setup mentioned above and everything listed on the page matches my actual settings.
Share Audio Devices is not ticked.
The default device is the onboard speakers at present.
Once the headphones are plugged in, they are listed as an additional option.
I agree, I will have to Save AS every session, so that I can go back to the other version if/when I hit this problem.
I hadn't done that because I haven't got anything significant done in Sonar so far.
I'll start a third attempt at creating a basic project this afternoon, but I'll check back here first.

The reason you run into silent buses is Windows is using the on board sound chip too or perhaps some other program is accessing it and changing its setup. Because of this Sonar can't see it and has no device to output to. The project is not in a state of error or corrupted. Once you restore the audio device it will play back just fine. 
 
You can use the on board chip if you must but you need to understand a few things about it. It will not give you low latency audio or the best sound. It will be noisy and full of distortion; distortion you may not even notice now but at some point you will. It will also limit you as to what sample rate you can use. What microphones will work with it.  
Sonar is a high quality DAW that needs good quality equipment to do its best. Also FL Studio will benefit too with a good quality interface.
 
This really should not be seen as an option but part of using a top quality DAW.
 
One more thing about weight and size. You have a choice of weight and size that will fit your needs perfectly. We are at a time where the available hardware is in abundance. Its a buyers market. Goto any music store that has audio interfaces for a computer and you will be dazzled by the selections. I take it you know how to use the internet. I would start there.     

You need one of these:
http://uk.focusrite.com/product-range/scarlett

Note the solo is designed to be portable.

Your onboard sound chip will not cut the mustard. Sonar states ASIO compatible, your chip won't be unless you are using ASIO4ALL which isn't a true ASIO driver anyway (and generally leads into trouble).

Whilst I thank all the contributors for their input, it really does look as though I've made an expensive mistake.
I tried the Sonar demo, which is next to useless in my opinion, having only the TTS1 and no save option, but was seduced by the prospect of a good range of instruments which suited my needs in the Platinum version, unlike Cubase 8 or FL 12.
I wanted a change from FL Studio after six years of compromises.
My problem with FL Studio is that I regard the instruments as inadequate for my purpose and of poor quality (except Sytrus).
 
Having considered all of your advice, it looks as though I will always have the problem of the silent lockout with Sonar on this notebook, so I'm going back to being productive on FL.
At least my old version of FL runs like a beast on my new notebook.
I'll try setting up Sonar's instruments and see what happens.
 
Please consider this post closed, thank you.

Good luck. But please note you can have the most powerful laptop in the world, however any DAW won't perform great without some decent audio interface. It also won't sound very good either. If you aren't having an issue with other DAW's you will eventually, you have a bottleneck.
 
You could try ASIO4ALL but if it does not work make sure you remove it. Generally not recommended unless it is a last resort.
 
You could also toggle the share audio devices setting:
https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=AudioPerformance.28.html
 
Also try disabling the chip in Windows Control Panel - Sounds.
Also do NOT run any other app other than Sonar when making music.
 
The link in my last post above shows a Scarlett SOLO which is Portable!

Cheers..

Graeme McTaggart
Whilst I thank all the contributors for their input, it really does look as though I've made an expensive mistake.
I tried the Sonar demo, which is next to useless in my opinion, having only the TTS1 and no save option, but was seduced by the prospect of a good range of instruments which suited my needs in the Platinum version, unlike Cubase 8 or FL 12.
I wanted a change from FL Studio after six years of compromises.
My problem with FL Studio is that I regard the instruments as inadequate for my purpose and of poor quality (except Sytrus).
 
Having considered all of your advice, it looks as though I will always have the problem of the silent lockout with Sonar on this notebook, so I'm going back to being productive on FL.
At least my old version of FL runs like a beast on my new notebook.
I'll try setting up Sonar's instruments and see what happens.
 
Please consider this post closed, thank you.

It will only be "an expensive mistake" if you give up on it now when you are on the verge of solving the issue.
 
If it's a Win10 system the latest versions of Sonar are optimized for that and get FAR better results using WDM mode than previous Windows and Sonar versions (based on benchmark tests done by the Bakers). So ASIO4ALL or an interface may not even be needed for light duty projects and/or if you learn how to mange system resources (freezing/archiving/bouncing tracks).
 
However this particular issue is NOT really a resource problem. It's configuration problem.
 
1) Turn off Windows Sounds
 
2) Go into your power management settings and set everything to "Performance" mode so that NOTHING on the computer is "falling asleep"
 
3) Disable/remove any and all startup programs and unnecessary factory installed bloatware on the computer
 
4) Find a low overhead antivirus that doesn't interfere with audio stuff too much AND add all the Sonar and all audio stuff to the AV's "Whitelist" so it's not scanning every action Sonar performs. MSE is the one us Win7'ers usually use but I'm not sure what's used for Win10
 
If you are unwilling to put in some time to find a solution then yeah... it is wasted money... but that's not Cakewalk's fault.
 
Also, call technical support if you can't get it sorted.

It do not think you're dealing with a SONAR problem, it sounds like Windows is taking over audio for some reason and therefore when SONAR looks for an output, there isn't one. 
 
I use SONAR a lot with a laptop when doing presentations and seminars; it has to be rock solid or my audience will get quite upset . There are some things you simply must do, like disable internal sounds. You also have to make sure the Sounds control panel is set up properly, with the desired device set as the default and you may have to reset this when you switch. You can also lose sound because the latency is too low (this would be solved with an ASIO interface).
 
SONAR and other professional applications want exclusive control over audio. ASIO has become the standard interface for professional software on Windows because frankly, Windows' onboard audio is there pretty much just for games, VOIP, and the media player. This is one area where Windows is waaaaay behind the Mac, whose Core Audio capabilities obviate the need for ASIO on OS X. Although Windows 10 is improving audio and especially MIDI in several ways, it's not up to pro standards yet although the consumer experience is better.

Thank you for this additional information Mr Anderton.

Doktor Avalanche
...You could also toggle the share audio devices setting:
https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=AudioPerformance.28.html

 
I was going to suggest this too, i.e. set "Share Drivers With Other Programs".

Also, if any of your existing projects still display the "silent hardware" message, look at the Master bus' output. If it is set to "None", set it to the Realtek.

Thanks.
When the onboard speakers are selected, they are numbered 1, the "none" option is not numbered in the list.
When the headphones are plugged in, they become number 1 and the onboard speakers change to number 2.
I don't think that is helping the situation.
I'm going to have to look at an Asio driver.
I've almost managed to input a single guitar track, but randomly, certain notes are not being played back by the Daw, probably a latency problem.
Then I tried to render an Mp3 this morning, oh dear, $499 and a license is not included, bummer. I have the answer to that in another thread though.