Thank you for your reply Davy.
The guitar app I've uploaded is just one of the many music apps I was trying to run on the N900. I believe that all of them relies on midi though.

As you were speaking about external players, I remember playing midi files on the N900 default mediaplayer after having installed the midi decoding package a couple of years ago.
They are talking about it in this thread:
http://talk.maemo.org/showthread.php?t=50301
Would that be useful for the aim of adding midi support in phoneme?

Even if maybe it could sound childish or simplistic to you, my programming knowledge is very limited and I can't help you as mach as I would.

Thank you so much again,
Luca