VLC/Quicktime/Windows Media Player AUDIO problem

First off, I have an HP Pavilion a1747c PC running on Windows Vista. The problem is that I have videos that can be seen perfectly fine on VLC Media Player, Quicktime, and Windows Media Player, but none of the audio is coming through!

At first, I thought it was the files themselves, but when I played some of them on my iPod touch, they were heard crystal clear. Next, I checked all the settings of all three applications. Check. The audio on my computer is working fine as well, because the sound effects of the PC and Internet sites have no problem streaming the audio (i.e. YouTube).

Does anyone how to fix this or even diagnose the problem?

P.S. I would like to include that when I check the sound mixer, the Web Applications and iTunes and other applications appear but my media players do not show up on the mixer. Could that help in any way?