a (simple?) question

[r . orfei]

As a lilypond rookie, I have a question to submit to this list:

I entered my music, tested it, and it (almost) sounds the way it should.

Now, how do I choose a different midi voice for my staves?

I saw a "Staff.instrumentMidi" thing somewhere but its syntax is unclear to me.

Can anybody submit a short excerpt, a hint or a direction (RTFM)?

I've already read the documentation but could not find a comprehensive MIDI 
paragraph.

... alas! while I am proficient in many programming languages, Scheme isn't one 
of 
them, so maybe the information is there, but I can't see it... ;-)

Ciao,
Ric