4 Properties are Scheme values. Most of them are in the generated documentation, but some MIDI properties are not. Here is their documentation
12 @item @code{midiInstrument}@indexcode{midiInstrument} @propertytype{string}
13 Sets the instrument for MIDI output. If this property is not set
14 then LilyPond will use the @code{instrument} property. This must
15 be set to one of the strings on the list of MIDI instruments that
16 appears in section @ref{midilist}. If you use a string which
17 is not listed, LilyPond will silently substitute piano.
19 @item @code{transposing}@indexcode{transposing} @propertytype{integer}
20 Transpose the MIDI output. Set this property to the number of
21 half-steps to transpose by.