Well... to tell you the truth I've never had much of a play with music ;D
I can tell you how I implement sound... I don't imagine music would be too much different
I use dBPoweramp with the .ogg plugin, and a program called CoolEdit to add effects.
As far as I'm aware, music should be stereo at a constant bitrate. I don't the the Khz matters all that much (though I'd make it smaller rather than larger, to keep the file size down).
I suggest try converting it from mp3 to .wav and seeing if media player plays it OK. If that's all good, convert it to .ogg and see if OFP likes it