O-K guys....I solved the problem.
It seems that for some reason, the previews are tied to Rhythmbox. Somehow, the volume in Rhythmbox had gotten turned all the way down. (How that happened, I'll never know, because I use VLC and Movieplyer....never Rhythmbox!)
I upped the volume, and now the previews work!!!
I guess Rhythmbox is tied to system sounds? Is there any way to change that? I friggin hate Rhythmbox- I had actually wanted to delete it...guess it's a good thing I didn't, eh?