On ubuntu/debian:
$ sudo apt-get install timidity
$ sudo ln -s /usr/bin/timidity /var/subsonic/transcode/timidity
Open Subsonic Web UI in browser and login as admin. Under Settings > General append "mid mod rmi rcp r36 g18 g36" under "Music files" field.
Click Save
Under Settings > Transcoding add new transcoder
Name: timidity player
Convert from: mid mod rmi rcp r36 g18 g36
Convert to: mp3
Step 1: timidity -EFreverb=0 -EFchorus=0 %s -Ow -o -
Step 2: ffmpeg -i - -ab %bk -v 0 -f mp3 -
Click Save
You can now stream MIDI files in Subsonic. Seeking/duration will not work as MIDI is rendered in realtime and ffmpeg does not know how long the track will be. Works on web UI and Android.