Cutting out the MIDI stuff from SoundSystemSmall was because of performance issues, not memory concerns.
MIDI is processed by Javasound. Problem is, Javasound seems to (poorly) handle MIDI events and audio clip playback from a single thread, and because of this, whenever the thread is busy processing MIDI events, any playing audio clips skip.
In SoundSystem, MIDI only works well when OpenAL is the active library. This allows Javasound to focus entirely on MIDI, while OpenAL handles all other audio playback.
Therefore, I removed MIDI support from SoundSystemSmall because OpenAL was removed. I may go back at some point to see if I can get both MIDI and normal audio to play smoothly at the same time in Javasound, but for now there was no point in including MIDI support. The small file size of MIDI music is useless if it can't be played at the same time as other sound effects.