The old Midi Kit, or midi1 for short, goes all the way back to DR8, when the BeOS only ran on BeBoxen. Fortunately for us, it is pretty well documented:
To summarize, there are four basic MIDI classes:
The following classes let you use the Midi Kit's General MIDI synthesizer:
To make MIDI data stream through your application, you create a "network" of BMidi-derived objects that send and receive MIDI messages.
The old Midi Kit is slowly fading into obscurity. You may want to use the new kit instead. Also note that the Haiku implementation of the kit sometimes behaves differently than the one from BeOS R5 or what the BeBook says, but usually for the better.
Especially the synth classes are not completely functional, but enough to play back General MIDI tunes. They should be backwards compatible with the majority of BeOS MIDI applications. Not all methods of BSynth, BMidiSynth, and BMidiSynthFile are implemented because some of them are rather obscure. BSamples is a complete no-op; in other words, with the Haiku Midi Kit you cannot push waveform data into the output stream of the softsynth.