Ancient template + MIDI = Percussion??

[Linda Cantoni]

I'm using Lilypond 2.6.4 (Windows XP native).  Using the ancient notation
template (Section 3.5.1 of the manual) as a guide, I reproduced a six-voice
version of "Sumer is icumen in."  The parts are marked Cantus I through IV
and Bassus I and II.  Except for a neomensural-style time signature symbol,
the notation is actually modern, in 3/2 time, and there is no incipit.  I
did not specify a MIDI instrument, assuming it would default to acoustic
grand.  But when I played the MIDI file, it came out as a variety of
percussion instruments.  I've tried forcing the default piano voice by
placing \set Staff.midiInstrument = "acoustic grand" variously in the global
block, in each of the six notes blocks, and in the score block, but no
matter what I do the MIDI always comes out as percussion.  (I initially had
a Staff.instrument for each notes block, but I replaced those with plain old
text markup in hopes of solving this problem.  It didn't work.)   

Here's my code (minus the actual notes & lyrics for brevity's sake):

\version "2.6.4"
\include "english.ly"

\header {
  title = "\"SUMER IS ICUMEN IN.\""
}

global = {
  \key f \major
  \override Staff.TimeSignature #'style = #'neomensural
  \time 3/2
  \override Score.SeparationItem #'padding = #1.5
}

cantusiNotes = \relative c'' {
  \clef "treble"
  ...notes...
  }

cantusiLyrics = \lyricmode {
...lyrics...  
}

cantusiLyricsTwo = \lyricmode { 
...lyrics...
}

cantusiiNotes = \relative c'' {
  \clef "treble"
...notes...
}

cantusiiLyrics = \lyricmode {
...lyrics...
}

cantusiiLyricsTwo = \lyricmode { 
...lyrics...
}

cantusiiiNotes = \relative c'' {
  \clef "treble"
...notes...
}

cantusiiiLyrics = \lyricmode {
...lyrics...
}

cantusiiiLyricsTwo = \lyricmode { 
..lyrics...
}

cantusivNotes = \relative c'' {
  \clef "treble"
...notes...
}

cantusivLyrics = \lyricmode {
...lyrics...
}

cantusivLyricsTwo = \lyricmode { 
...lyrics...
}

bassusiNotes = \relative c {
  \clef "bass"
...notes...
}

bassusiLyrics = \lyricmode {
...lyrics...
}

bassusiiNotes = \relative c {
  \clef "bass"
...notes...
}

bassusiiLyrics = \lyricmode {
...lyrics...
}

\score {
  \context StaffGroup = choirStaff <<

    \context Voice =
      cantusiNotes << \global \cantusiNotes  >>
    \context Lyrics =
      cantusiLyrics \lyricsto cantusiNotes { \cantusiLyrics }
    \context Lyrics =
      cantusiLyricsTwo \lyricsto cantusiNotes { \cantusiLyricsTwo }

    \context Voice =
      cantusiiNotes << \global \cantusiiNotes >>
    \context Lyrics =
      cantusiiLyrics \lyricsto cantusiiNotes { \cantusiiLyrics }
    \context Lyrics =
      cantusiiLyricsTwo \lyricsto cantusiiNotes { \cantusiiLyricsTwo }

    \context Voice =
      cantusiiiNotes << \global \cantusiiiNotes >>
    \context Lyrics =
      cantusiiiLyrics \lyricsto cantusiiiNotes { \cantusiiiLyrics }
    \context Lyrics =
      cantusiiiLyricsTwo \lyricsto cantusiiiNotes { \cantusiiiLyricsTwo }

    \context Voice =
      cantusivNotes << \global \cantusivNotes >>
    \context Lyrics =
      cantusivLyrics \lyricsto cantusivNotes { \cantusivLyrics }
    \context Lyrics =
      cantusivLyricsTwo \lyricsto cantusivNotes { \cantusivLyricsTwo }

    \context Voice =
      bassusiNotes << \global \bassusiNotes >>
    \context Lyrics =
      bassusiLyrics \lyricsto bassusiNotes { \bassusiLyrics }

    \context Voice =
      bassusiiNotes << \global \bassusiiNotes >>
    \context Lyrics =
      bassusiiLyrics \lyricsto bassusiiNotes { \bassusiiLyrics }
  >>

  \layout {  
    indent = 0.0\cm
  }
  \midi { \tempo 2 = 120 }
}