How to convert midi into mp3 while adjusting channel level?

[maaaaaaaartin]

I have a midi file with multiple channels (say bass, tenor, alto, soprano). How to produce multiple mp3s with different volume distributions (say b=+0, t=+0, a=+0, s=+0; b=+0, t=-12dB, a=-12dB, s=-12dB; b=-12dB, t=+0, a=-12dB, s=-12dB; etc) in an automated way? Sorry if it is a trivial question. Thanks in advance for any help!