No Audio Except Thru Sonar

I assume you're using ASIO? If so that would explain it. IIRC you have two options...
 
1. Run SONAR using a Windows driver mode, like WDM.
2. Don't know what version of Windows you're using, but go into the Windows audio settings and set the Focusrite as the default for Windows.
 
Circle back and let me know if that helps.
 
Remember that the big advantage of using Windows drivers is that, like the Mac's Core Audio, they can aggregate interfaces and run sound for different programs simultaneously. However ASIO will usually give a performance edge (YMMV) with high-performance audio applications like SONAR, so you have to decide which is more relevant to your needs.

Did you try what I suggested in (2) above? Much to my chagrin, Windows often resets defaults after an update. 

I haven't worried about this issue for, well, ever. I use two audio systems. One is for Sonar and other programs that can use a high quality audio interface and the other is for everything else. That one uses the on board audio chip and is quite adequate for those purposes. I've been doing this for so many years I can't say when I started doing this. I use to use a Sound Blaster along side a Tascam digital mixer. That proved to be a very simple solution to the problem of switching on and off my Tascam. Plus the TDIFF card I used had trouble with hanging Windows if it was on when I booted Windows. It became a habit that has proved useful through the years.
 
You need to invest in a separate setup for your speakers but once its done it should not need further upgrades. The other benefit is I don't need to have all my external gear on just to read the Sonar forum.      

@John, That would sure solve this PITA issue. It's just frustrating that it has worked flawlessly, up until today. The other issue that would be resolved with two systems is my monitors are powered, and they shut off when the don't thinks there's sufficient signal being supplied. Anyway, I just so happen to have another set of speakers....

@SuperG, Did that, and it's set correctly. Thanks for the right-click hint. I always went in thru the Control Panel. :)

That's a great hint, @rickidoo. In part of the troubleshooting process, I set the sample rate to 96k. However, when I checked in now, it was at 41 k. Strange
 

John
 
Exactly! There is another issue that this approach solves. Windows will change the sample rate depending on the source. This is a problem if you also use the same setup for your DAW. 

Some manufacturers (i.e. RME) don't recommend using their audio interfaces as the default Windows sound system/driver.
^^This is one of the reasons.
I also use a second sound card/interface as the default for Windows and never had any problems.
So, trying to use the onboard chip exclusively for Windows is not a bad advice, I think.
 
Best,
Klaus
 

Makes perfect sense to me. Thanks for the great advice. Another bonus is I can power-down the 18i20 between sessions. Thanks again!!!

woodenmusic
And finally, Realtek and Focusrite existing side by side. It's a wonderful thing!

That is very good to hear. I'm so glad you found a way to do this. And yes I do that too. Power down what I'm not using. 

The IRQ should not pop up as an issue (famous last words), but what I have seen is Windows reassigning the "default" as it sees fit unless one is specifically assigned as default by the user (at least in Win7). The way to do this is to have both devices online and visible to Windows; and even if the correct one is default (Realtek HD) in Windows Sound Card properties, assign the other (audio interface), and then assign the Realtek HD again... that way the user explicitly chose "Realtek HD" as the default and it should stick from that point going forward.
 
After doing this, Windows should never "take control" of the audio interface on you, just make it available for use.