Maybe you can strip out the Audio as a separate track. Clean it up and punch up the gain in a Audio application and replace it in using something like Final Cut Express.
Is this surround sound? If it is, there is probably settings for certain speakers (mine has master and rear). If so, one or more of the levels is probably off.
Firstly you need to check your volume control from control panel, if that is ok so there is some problem with your sound drive. If both are working fine so you need to download that movie in some other format.