Personally I don't see why would someone want to convert from ac3 to mp3 provided that xvid/divx movie was derived from a source with surround sound.
If you are having problems with the ac3 sound being too low I suggest opening up the ac3 filter (if you're using that) in control panel and increase the gain by +20dB.
This is what I did and it saved me turning up the volume hehe. You can grab the AC3 filter at
http://www.free-codecs.com/ if you don't already have it.