If anyone is having this problem, it's because the MP3 player in question transfers in MTP mode. The PS3, along with car stereos with USB and AVRs and pretty much everything out there, uses MSC transferring. If your MP3 player allows you to set the mode, such as Sansas all do, you can get it to work as a normal USB key with your desired output device.
Creative and Apple, for example, do not allow MSC.