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