Set midi volume per voice?

[ghe]

Hi list,

Is it possible to set the volume of the midi tracks for each different 
track?

Till now I have:

one = { a b c d }
two = { d e f g }
global = { \key b \minor }
\score {
  \new Staff <<
    \new Voice = vone { \voiceOne << \global \one >> }
    \new Voice = vtwo { \voiceTwo << \global \two >> }
  >>
  \midi {
    \context { \Staff \remove "Staff_performer" }
    \context { \Voice \consists "Staff_performer" }
  }
}

This generates a MIDI file with two tracks, 'vone' and 'vtwo'.  I know 
that it is possible to get one of the tracks play louder than the other 
with for example:

one = { a\pppp b c d }
two = { d\ffff e f g }

But is it possible to do this in the \score definition, that is, without 
touching the definitions of 'one' and 'two'?

Thanks!

--ghe