Csaba2, Some players play both formats. But a lot of people report having problems with MTV sound.
http://www.mympxplayer.com/viewtopic.php?p=6135#6135
The only way you could change format is to change your firmware and the chances of you finding the right one to suit your player are not good. Changing firmware is very dangerous and can kill your player if you get it wrong.
Can you upload a sample of your AMV to
www.ezshare.de so i can download it and see how bad the sound is.
If you are looking for top quality stereo video on these players you are not going to get it they are just not powerful enough. you need to spend a lot more money on something like an iPod Video or something that plays standard video formats.