Midi question

[Thomas Scharkowski]

Hello,

I have noticed recently that lilypond produces "empty" midi-tracks 
sometimes. I have already tested several *.ly files I had written 
some time ago (with version 2.0.1, they did produce correct midi-
files then), they do not work anymore (for midi)!
I have tried some examples from the doc examples (Bach Fugue and 
Satie) and these still _do_ produce midi.
So I am a bit confused and perhaps I have missed something simple, 
but I cannot find out why midi does not work anymore for my files.
I have a short example here, pleas tell me what is wrong:

Thomas
cygwin
--

\version "2.1.28"
\score {
\notes \relative c' {
\repeat unfold 16 {
        c d e f 
        g a b c 
        b a g f 
        e d c b }
        c1 \bar "|."
        }
\midi {\tempo 4 = 96}
\paper {}
}