AC3 filter, a codec which you need to play Xvid/Divx movies with AC3 (6 channel) sound.
It is also a post processor which you have to use if you have surround or 5.1 speakers. It can create 6 channel sound out of stereo. Be sure to choose the correct speaker configuration for your speakers in AC3 filter configuration and in Windows. Without AC3 filter, forward speaker channels are just mirrored to the rear speakers. AC3 filter truly extracts the voice and puts that on the center channel.