We will assume that you have checked the obvious, such as volume in your player and other sound settings on your PC.

Most of the time this problem is caused by presence of AC3 audio (almost always on 2CD movie releases) stream and absence of AC3 filter that is needed to decode it. How to solve this problem?

1. Grab AC3 filter (it's free) from here: http://ac3filter.net/ Install it, normally a reboot won't be required but it's worth it, and try playback again. You should now hear the audio that you didn't before.

2. An alternative solution would be to use a player that already has AC3 filter included so you won't need to download it separately. The most popular such player is VLC player, which can be downloaded from here: http://www.videolan.org/vlc/

If, after trying both of those suggestions, you are still having issues with the audio then please let us know through Contact Form (use "Other" category) and don't forget to include the link to the post in question.