did you recently download a movie in the .avi format and can't hear any sound? this may be due to a couple of missing components that do not ship with a typical install of quicktime player. most commonly, I have seen movies that have the sound encoded in AC3 format.
make sure to clean out any other ac3 and sound components in /Library/Quicktime/ before installing the new ones.
STEP 1
Download and install the latest free version of the DivX codec
STEP 2
Download and install the latest version of A52Codec - 1.7.7
Good luck.
