relative volumes in MIDI output of choral music

[Dong Hoon Van Uytsel]

Hi Lilypond users,

Hi, this is a newbie talking.

I would like to prepare MIDI files from a Lilypond file describing a 
choral piece to help my singers study their parts at home. Now I did not 
readily find how to adjust the relative volumes of the MIDI output tracks. 
This would serve to accentuate a certain parts, while the other parts are 
not completely muted.
Is there an easy way to achieve this?

Thank you
Dong Hoon