You can package Timidity with your project if you follow the license. You can also use Timidity to convert midi files into WAV, or OGG files. This would only be useful for the NodeWebkit, and you would need to pass commands via CLI.
I hope if Chrome adds Midi they also include some sort of MIDI driver for crappy computers like mine.
@tatogame, your game is amazing, could I possibly have a look at the MIDI plugin you made? Is it possible to do the reverse, let's say I build a keyboard and then use that to send midi signals to a DAW like Ableton or FLStudio?