Hi pxp.
Have a look at this post here:
There are TWO (2) Different MTV formats!.
See if that converter mentioned in that post works for your player.
Can you also look at UPLOADING the two sample .MTV files you have on your player? We can then take a look at it and see what we can help you with. You can upload it directly into this site here:
Downloads - MyMPx.
Let us know if that converter helps.