Sony Vegas (home studio) problem

Hopefully someone here might have an idea about how to solve this (probably simple) problem. I have Sony Vegas Home Studio 9.0c, and have had it for about a year. I've never had any trouble from it, but loaded it up today after months of it lying dormant, and now I get no sound from any file playback.

I can drop a video file into a video track, and after it builds the audio peaks, the audio for the video file shows up under the voice track. But when I click play to preview the file, i get video preview, but no sound at all. I've tried several different file types (avi, mpg, mp4, etc), and even tried a new blank project with only an mp3 loaded into a sound track, no video, and still no sound. It's as if the entire application is muted. The same files play sound fine in VLC, foobar, etc.

I've searched the interface, the various mute controls, and nothing seems to be muted, and the volume bars are substantially up. The same files will load and play audio in Windows Live Movie Maker.

Longshot maybe, but does anyone have any suggestions? Sony wants $30 to talk about it on the phone

edit: tried uninstalling and re-installing the program, no luck.

I downloaded the Home Studio version 10.0 trial, and it *does* work. This is all weird considering 9.0 worked fine for a long time, I did nothing to it, made no changes, still on the same OS install, and it just suddenly stops working.