I was starting to lose interest.
So then I wrote a whole new synthesiser from scratch followed by enough of a midi sequencer to get a midi of Lazy Jones working - and I think it sounded better than the bundled Java midi synthesiser (more hard/harsh just like the original). It uses a thread to render a frame of samples at a time based on a master clock and uses a simple 'runLater()' style mechanism to synchronise parameter updates on the various components so threading issues are just hidden.
I tried a few other files and none worked either at all or very well but after another day of hacking I've got fully polyphonic voices and adsr envelopes and so on and so forth. Most midi files apart from Lazy Jones still sound kinda shit because I don't have any real instruments or the midi processing units (and well, it's midi), but they do play with the right timing and tone (I think, hard to know since midi files usually sound a bit out to me). For efficiency I created a multi channel oscillator and envelope bank which works pretty well but limits the instruments to simple waveform + ADSR envelope.
Most of it was pretty straightforward considering i just did it off the top of my head but I found I had to do some filtering on the amplitude changes to avoid pops when using a sooth waveform. One problem I have is that square and sawtooth are just so much louder than triangle; some filtering seems to help but I guess that's just the way it is and it's up to the musician to compensate.
Probably wont get much farther than that but it was a fun few days. It can be made an interesting real-time problem and parts are paralellisable so if I was keen i'd probably try some epiphany code but i don't think i'm that keen and it's fairly light cpu wise anyway.
I guess I forgot about the space invaders sfx. If the latency is acceptable i might try just using my synthesiser as a real-time sound mechanism, maybe maybe.