Below is a link to a recent thread that refers to audio quality adjustment:
http://www.mympxplayer.org/viewtopic.php?p=22641#22641
Also, someone recently posted a modified AMV v4.00 converter that allowed for selection of the otherwise greyed out 32k bps single channel audio option. I believe the file was labelled AMVConverter_4_mod.zip though I couln't locate it on the site just now....