The original divx 311 alpha audio codec ('divx audio' or something like that). At the time the only MPEG layer 3 audio codec avaliable would often times result in loss of a/v sync so it was the lesser of two evils. Most people who run into this problem are relying on something like xvid or ffdshow to play back divx311alpha / "divx3" files. Nothing wrong with that except that you still need to install the original divx311alpha codec in order to get the audio.