Suddenly, the volume in my studio is about half normal.  This applies to speakers connected to the V-Studio, headphone connected to the V-Studio, **AND** to wav files just played from my PC.
 
Is there something that could suddenly make the volume output of my PC much lower?
 
UPDATE:  OK, in the Windows control panel, I found VS-700 as the main playback device and the volume was turned down to 20%.  I put it back to 100% and I'm OK.
 
BUT...how the hell did that happen?
 
 

Some programs have a ducking feature built in which often controls the Windows sound mixer (globally). Do you remember programs you ran before you noticed this?

If the VS-700 is your main Windows sound device, all audio applications will use it. So if, for example, you are watching a YouTube video and you turn down the sound (in Windows), that will affect the sound level in other applications, such as Sonar.
 
If your PC has onboard audio, you could use that for everything except recording with Sonar, to prevent this from happening (if in fact this is what happened).

Is it possible you accidently toggled the consumer/pro impedance settings on your audio interface? My Echo Layla 3G has a +3db/-10db switch (or similar) that can have a dramatic effect on audio volume, completely outside of Sonar...