Quicktime X is a great little piece of software. However, Quicktime X is very finicky on which codec/file format it can decode/play.
Luckily, there are DOZENS of alternative video/media players and extensions available for the Mac. Some of them are even FREE. Some of the better FREE ones include...
PERIAN (quicktime extension)
VLC Media Player
Clips where the audio doesn't play, show: Linear PCM, 16 bit little-endian signed integer, 32000 Hz, 4.0 (_ _ _ _)
This indicates the file was encoded with 4 channels of audio but none of them have been actively assigned to an output channel. If you have QT 7 Pro, open the file in the "Properties" window, select the audio track, select the "Audio Settings" tab in the lower half of the window, and set the proper output channel assignments.