I'm not using high bit rates, and I DO close SONAR and still have this problem.  Unplugging and plugging the TASCAM didn't solve things and sent me a number of error messages (didn't memorize, but along the lines of "Audio engine has stopped"), so I had to reboot anyway. 

Sonar and Windows system audio are two totally different thing as im sure you know.
It definitely sounds like a driver issue to me, like your ASIO driver is holding your Windows system driver hostage.
For me anyway... I can open You tube while im working inside of sonar and play both if I choose.
Sometimes I even use Windows media player or You tube to watch/listen to material while working on a project inside Sonar. This is of course because I have 2 sets of speakers. One for Windows system audio and one plugged directly in to my Audio interface. That last part is critical.
 
I would check the driver your using. This does sound like a classic case of ASIO4ALL. Which is a wrapper for Windows System audio. Uninstall your Audio driver for the Tascam what ever it is and look for a new update or reinstall what ever you can find online for your device.

Set your audio device in your DAW and your projects to match the Windows audio sample rate of 44.1k and see if you can go back and forth OK.
 
I have observed that if a DAW using ASIO is set to use a sample rate higher than what Windows is using, the audio interface must switch to the higher sample rate to accommodate the DAW session.  That will silence Windows audio, because the interface can not support both sample rates at the same time.  It is either one or the other.
 
If for some reason the audio sample rate is not resetting to the Windows default after ending the DAW session, then setting both sample rates to 44.1k may be a functional workaround.  That is what I had to do on my system.

Here are my steps to resolve stuff like this:
1) Open up Task Manager and make sure SONARPLT.exe isn't lurking in the background after you closed it. Force the task to end in Task Manager if still exists.
2) Unplug the USB cable for the interface (doesn't matter which end). Wait a few seconds. Plug it back in.
3) If all else fails, reboot the system.