It seems to support 4 simultaneous notes at a time, using the computer keyboard (e.g., press down zxcv together), but if you hit a 5th key it's ignored.

For key velocity (is that what you mean by instrument expression?) -- there's maybe not much point attempting to tackle that when it's not emulating instrument voices yet anyway, or supporting MIDI input. You can't exactly capture key velocity from a computer keyboard.

There's some mention of gain in the code snippets, at any rate -- I think each voice may have its own "volume knob", in effect -- but I haven't dug into it at all.