check the audio format of your avi (for instance with nandub [file->file information], or using
AVIcodec)
if it is ac3, you'll need ac3 filters
download audiofilters.zip from
doom9's
if it is divx audio, you'll need to install divx3.11 (I think you can find audio only installation of divx3 in some codec packs)
if it is wma audio, you can find wma8 codecs from microsoft
...