The question now is how and to what convert. I see the manual, but if you say that audio needs to be converted separately, I want to know how to convert audio to be used to this process.
Export the audio stream as wav file (You can do this with VirtualDub, go to "File" then "Save WAV") , export it to VP6 avi without audio (Select "No audio" from the "audio" menu), then run the conversion with SX like this:
sx -sndstream -eaxa_blk source.wav -=source.asf
Change "source.wav" and "source.asf" to the name of the audio you just exported from the video file.