Hello!
The best and
free MP3 Player that I tested
and took with me all the time is
GSPlayer.
Give it a run! It's wonderfull!
http://hp.vector.co.jp/authors/VA032810/
And you can of course assign controls to a hardware button...
Version 2.0 is out

always the same features with some
corrections: MP3/Ogg/WAV playback, Shoutcast support,
equalizer/surround/bass effects, skin support, hardware
button mapping, noiseless playback, and even a sleep timer.
@ bientot

,
Joshu.