What we're trying to do here is narrow down whether the problem is:

a) In the MP3s you have created.

b) In the player itself

c) In the amplification and speakers

d) In the way the player's audio is configured.

By making sure the files don't sound like that when played on your PC, then you have eliminated (a). Then we can move on to something else. It doesn't have to sound "great" on your PC, it just needs to play without the distortion you described.