Video but no audio

The videos that I have been downloading (.avi or xvid) movies give me video and only background audio. The music in the background and the background noise plays, but the voices of all characters does not play. It is very frustrating and I have no idea why it is doing this? Can anyone help?

You probably turned on karaoke mode in your audio driver on accident. Happened to me once, it was interesting as hell when I was listening to music with an echo effect on the singer's voice: you could only hear the echos, but not the actual singing.

how many speakers do you have? make sure your audio settings (probably in your sound card's mixer settings somewhere) are configured to output to the proper speaker config.

chances are the movies you're trying to watch have a surround track embedded into it, and as a result, if your mixer settings are set to 5 or more speakers, it's going to be trying to output to a center channel speaker when you don't have one, and you're not going to get dialog.