Why does my Start Navigation sound keep disappearing?

Ever since I've had Vista (about six months), my Start Navigation sound event
(which I really like hearing the feedback from as I go from folder to folder
in Explorer, and from web page to web page in Internet Explorer) just
disappears.

I have to go into the Sound control panel and re-enable it, even though I
have saved its setting in the My Sounds scheme. When it goes away, it is
also then missing from the My Sounds scheme.

Sometimes I have to reset it a couple of times a week; sometimes I can go
for six weeks without a problem - but, then, out of nowhere (in the middle of
doing something mundane), the sound just disappears again.

I'm using Vista Home Premium on a Dell Dimension E520 desktop with onboard
Sigmatel STAC92xx C-Major HD audio.

Any ideas?