This sounds like an artifact of high compression to amv format. 1st-- Install media player classic. Google it. Then install PocketDivXEncoder. This is step 1 in converting your avi, mpg etc. Choose settings that most closely approximate your mp4's resolution ie. 128x128, at 16 fps etc... select sound settings. link :
http://www.pocketpcfreewares.com/en/index.php?soft=972
After resizing video and audio, then convert w/ amvconvert.
Post your results.....