In the last few days, I integrated the FluidSynth library with our own MidiKit. It seems to work good when I test with MidiSynth on Haiku. Though I had to download a GM soundbank for this (I used sYnerGi-8Mb.sf2).
All midi kit is now included on the Haiku image. Hopefully some of you can test and find bugs on the midi/midi2 kit. Feel free to submit them on our Bugzilla.