Then it probably has something to do with the way the jack & plug mate - or possibly in either the handset's audio configuration (not sure what to do about that) or perhaps the music was only transferred on one channel.

Assuming the music is on an SD card, try listening to it on a computer.

If you hear both channels on a different player, you may have a bad connector, a firmware glitch or a blown audio pre-amp or amplifier channel on the handset - may be time to see about a warranty replacement