Nah, mine are mp3's, wma's, and it even picks up my ringtone folder (use the card in my phone sometimes) and they're encoded to like 64kb and 96kb quality and they play fine.
I've noticed it's a very random thing. Some will finish one day, the next day, that very same mp3 will cut off short. It really doesn't make any sense to me honestly.
I bring the card back to the computer and play it, and it always finishes, so it's definitely something in the unit that's not reading something right.
Sadly I've given up though.. I boxed it back up yesterday and I'm heading back to Best