In Sonar preferences under devices, try disabling everything except for the audiobox (if there is anything).

Sonar
Pref make sure ASIO or the right driver you want to be used is still selected...might want to not share audio device in that pref page settings...
 
in MS the default audio has ins and outs also...perhaps something got muddled in that?
 
I set my default to onboard audio, and leave my other audio device free...and only my software for music uses...which also have in outs in MS audio settings to check and make sure they are correctly set...line out..not phones or spdif..kind thingy....hope it routes out right...if not try reinstalling driver for the audiobox....hope this helps...if the selectable ins and outs do not show, then something ain't right with something...

There is nothing else.

One other thing to make sure - close all the other programs (iTunes, etc.) including your browser and then open Sonar and see if it will play audio.  Sometimes those other programs will hold your interface for ransom.

Unfortunately, I'm at work, so I'm not able to get into sonar to stir up any more ideas right now.

oh,,if you use the audio usb box as preferred default, you may have inadvertently reset buffers and or bit rates when booting...sounds played are not the same bit rate, the audio card changes to, and locks in on..then you open Sonar project, and nothing plays...because the audio device cannot switch bit rate or buffer size to accommodate the project.
perhaps?
...that is why I do not use my audio recording device as MS default audio...and I turn off all MS sounds, that play when booting or shutting down, clicks warnings and such...just to make sure!

I'm considering uninstalling and reinstalling the software, if anyone thinks that will help.

if it is a hardware issue, reinstalling will not solve it...open your audio device applet and check to make sure the bit rate and buffers are right for the project you are working on....make sure, then open sonar..see what gives first.

also usb ports, in devices in "device manager"..open each one, and select always on in their preferences.

the Audiobox applet is set normally, and none of the USB devices (which actually aren't in Devices in the Device Manager they're in Universal Serial Bus controllers) don't have a "preferences" or a "always on" option that I can find. It doesn't seem to be a hardware issue because all my other audio is fine, I can play my keyboard, listen to mp3s, I get audio through my browser, even other DAWs, just not Sonar. I really appreciate your input but if anyone is out there that is actually at a computer and can help me run this down I'd appreciate that too.