I use 8 bit oscillators, each with 64 possible volume levels. I currently make the maximum total oscillator volume (sum of all the volumes) be 64 - if several oscillators are full on, it lowers their volumes so that the total is exactly 64.
After the resonant filter code, it multiplies everything by 4 (the filter requires some headroom to work) with clipping in case the filter is resonating strongly.
I've just fixed the link to the code on my web page, so you can see how it works:-
Also, a new video - now has pressure sensitive pitch bend and filter modulation touch sensors too (also made out of random bolts I had lying around).
I don't have any hardware low-pass filtering, as the 32khz sound is filtered out in my lm386 amp which apparently has pretty much no response above 20khz (for that matter, I can't hear anywhere near 32khz anyway). I just bung it straight off pin 11 -> pin 3 on the lm386 amplifier, then straight to the speakers (oh, there's a couple of capacitors & resistors in there to remove DC bias and to lower volume, but nothing clever.