All we got over here is bacon flavored condoms.

How do you know they're bacon flavoured?
 

You can smell it as soon as you tear open the package.

ampfixer
How do you know they're bacon flavoured?
 

You have a dog hanging off your...
 
Nevermind. 

Welp...today I had some time to really troubleshoot this and it didn't take all that long to find and fix the problem. Turns out...natch...I was wrong: something had been changed. How or by who is still a mystery. But the Windows "wave" volume slider was down. Simple as that. The mystery is I am the only one currently who uses that box and I have never, ever opened up that dialog and changed the position of that fader. And, if they are to be believed, neither has anyone else here. Go figure. In any case it's all fixed now. I figured it would just take a little sleuthing. Sorry Bit, had to cancel your room at the Embassy Suites.

There are quite a few applications that will reset the Windows volume without any human assistance. if any programs were run on your computer that might explain this, even though no one would have any recollection of manually making the change. Communications software like Skype etc. can be instructed to do that via a setting in system sound under communication tab, but games and other applications that you would not expect to do anything often do.
 
Google: windows 7 software resets volume 

You are probably right...but remember, this is an XP box.

At times I find Windows sound muted w/o anyone touching it, so anything's possible.