Midi file is quiet???

[tim]

I have an engraving of a Vivaldi piece that has a ChoirStaff with three staffs 
within it, then a 
separate staff, then a PianoStaff with the normal two parts. The last two lines 
in the \score 
block are:

  \layout {}
  \midi {\tempo 4 = 70 }

Then a curly brace to end the \score block, and the file ends.

The PDF looks absolutely beautiful;, no problem there. But the midi file is 
completely quiet, 
from beginning to end. What would cause that?

Thanks!

-- 
Tim Slattery
tim@risingdove.com