midi files themselves don't contain any audio/music. They contain just midi messages (note on/off, volume up/down, etc) that are sent to instruments. so to actually play a midi file, you need to play it on certain instruments.
Something like timidity (
http://timidity.sourceforge.net/ ) can produce audio when playing those files because it contains instruments. Other programs try to play midi to your midi out (as found on some audio cards), and can be used to control for instance keyboards, synthesizers and drumcoputers.
Bottom line is, that how a midi file sounds completely depends on what you play it on/with. For instance, something that I made on my computer with my synthesizer and drumcomputer attached might sound great here if attached to that hardware, but might not make any sense at all when played on a
general midi keyboard. Just because that contains completely different instruments.
If you want to play a midi file that was made with "general midi" in mind, try playing it with timidity++. I haven't tried it myself, but I understand it's more or less the default midi player for linux. You should be able to get it with
slapt-get --install TiMidity++
I hope somebody else can help you set it up if necessary.
