Drums & midi not working properly

[Sebastiano Vigna]

I know, this should submitted to lilypond-bugs, but I had no chance in
subscribing up to now. The following source


\score {
\new DrumStaff <<
       \context DrumVoice = "1" { s1 }
       \context DrumVoice = "2" { s1 }
       \drummode {
         <<
           { r4 sn r sn } \\
           { bd r bd r }
         >>
        }
    >>

    \midi{ \tempo 4=80 }
}

generates no notes in the MIDI output (it is almost exactly copied from
the manual). Suggestions?

-- 
Ciao,

                                        seba