Sounds can disappear, but not reappear

Hello, and thank you for reading my post.

Today, suddenly, some sounds I always had (such as "Windows XP Start.wav"
for "Start Navigation") weren't there when I expected them to be. (Yes, the
file IS on my hard drive, but the sound itself wasn't happening when it was
supposed to.)

Puzzled, I explored further and tried to re-select that sound in the Control
Panel's "Sounds and Audio Devices", and "Apply" it, but, unfortunately, even
though it "looked" like it was selecting a sound, the second I tried to click
somewhere and navigate (such as within IE), expecting to hear the sound, it
still wouldn't be there AND a review of that same area again in Control Panel
showed that my choice was gone once more.

Also, if I tried to choose a sound scheme that used fewer sounds than I had
to start with, THAT would work but I couldn't then re-add manually those
sounds I'd now de-selected. (I hope that makes sense... )

Please advise what I can do about this soon (yes, I'm fully updated via
Microsoft/Windows Update), and thanks again.

Sincerely,
Bram Weiser