You may want to compare the driver mode Sonar is using and what drivers are available in the projects that have problems compared to ones that play ok. My guess is that there was some audio driver conflict when opening the project and Sonar reassigned the drivers or channels based on what was available. Maybe it even assigned the project to nothing if nothing was available? Perhaps you just need to reassign the driver mode for these projects and/or audio channels.
 
On a side note, I always found Windows hardware drivers to be a PITA in general. That's why you will see the constant replies like, is your software up to date, turn off the network adapter, update your graphics card driver, don't run other programs at the same time as Sonar, get a dedicated audio interface, etc. I have zero of these problems on a Mac laptop. And the only reason I mention that is because I wish Windows and PC hardware manufactures would get their *&^* together.
 
But if latency is not an issue for you using the onboard audio supplied on your laptop with Sonar, it's most likely just a combination of getting the driver set up correct in Sonar and learning what not to do to keep Windows/Sonar from freaking out by avoiding a conflict.
 
Why FL Studio would be more stable than Sonar is interesting though. Perhaps Sonar is using a different driver mode than FL Studio. Maybe look to see how FL Studio is set up as far as drivers. Perhaps Sonar could be set up the same.
 
As far as providing MP3 export option in Sonar, yeah, you need to buy or configure Sonar to use a free one. I've heard all the responses in the past about whey Cakewalk does this. But seriously, I don't know of any other software that makes you unlock MP3 export. Seems like it would be easier for Cakewalk to finally just build this in to the cost of the program if that's what everyone else does.

I'm currently using MME set to 250ms.
I'm going to try WASAPI later today with a similar setting in terms of samples: 11360/258ms which is my FL specification.
I'd like to avoid Asio, which I've never needed in the past, especially as I don't record anything.
 
Basically, if I start a new project with headphones plugged in, NEVER open it without the headphones plugged in or it's dead! and that's permanent.
The opposite also applies, so I have to have ext. speakers or headphones plugged in if I'm going to use Sonar.
At least that's one problem solved :0)
 
FL has three options: Primary audio driver, Headphones, Speakers.
Left on Primary audio, it switches on the fly to whichever of the other two is being used.
It also has Mp3 rendering built-in and an End of Project marker.
 
The whole situation reminds of when a certain software manufacturer would release a new version of a familiar programme, but with new names for familiar things and a new menu structure, just to confuse the users and drive them towards paying for a training course.

Graeme McTaggart
The whole situation reminds of when a certain software manufacturer would release a new version of a familiar programme, but with new names for familiar things and a new menu structure, just to confuse the users and drive them towards paying for a training course.

Don't expect to learn a DAW without reading or studying documentation. Or watching videos. Sonar is a virtual recording studio. Recording isn't something you just pick up in 5 minutes. If you are not prepared to learn tool or have barriers learning new things maybe it's best to stay away from DAWs or any new software. Cheers...

And again this is not a Sonar issue.

Sounds like you know what you are doing. It's probably just some playing around with the settings in Sonar getting it configured best for your laptop and seeing how Sonar handles the hardware drivers of the sound chip in terms of sharing or not sharing with other programs. You may just need to do things consistently, like you mentioned about plugging in the headphones before launching Sonar, etc.

Graeme McTaggart, let's take 10000 Sonar's users. 99900 of them use ASIO, 99 use WDM or WASAPI, and 1 use MME (you). It is only my estimation, but you can understand what I'm talking about. It is hard for us to help you in your troubles.

Well, in an earlier post (different thread), the MME option was recommended to me by a Sonar user, that's why I stayed with it initially and I don't know 10,000 other Sonar users to make a comparison with.
This is the problem of learning by committee, although all advice is well intentioned, some of it more useful than the rest.
I am working my way through the available options and testing things out.
 
I'm not going to respond to the "takes more than 5 minutes" comment above.
That doesn't help or even contribute to the discussion.

In my small experience the only way to make it work with build-in audio is ASIO4ALL. But it is only short time decision because of low sound playing quality.

Graeme McTaggart
I'm not going to respond to the "takes more than 5 minutes" comment above.
That doesn't help or even contribute to the discussion.

Exactly what I thought when you wrote the below, so apologies for getting a little cheesed off.

Graeme McTaggart
The whole situation reminds of when a certain software manufacturer would release a new version of a familiar programme, but with new names for familiar things and a new menu structure, just to confuse the users and drive them towards paying for a training course.

So let's just concentrate on your issue shall we?

This snip from the details of the new insider build 10547 of windows 10 may be of interest.
 

• We have fixed several issues with audio – including issues specific affecting Realtek audio devices.

I know I will get a mountain of flak for this but try ASIO4ALL.
I am no fan of it but it solved a similar issue on my I7 laptop.
I wanted to use Sonar while travelling on biz and not have to lug around another piece of gear (interface).
I think the OP has a valid point. I tried for the better part of 2 days to get Sonar Plat to work with the Conexant chip on the laptop. Even MME had serious issues. WDM, WASAPI did as well.
I got other programs to work fine even up to 192Khz but not Sonar.
I had almost given up and decided to try ASIO4ALL. Amazingly, it worked like a charm.
I got the lowest latency I have personally seen...like 3 ms or so @ 44.1.
I asked the following question in another post a while back.
Why can a second rate free program like ASIO4ALL take the ASIO from Sonar and convert it to  WDM at very low latency and work with the chipset and Sonar cannot ?
Until someone from Cakewalk explains this (in detail) I have to at least partially agree with the OP.