Well, I have a lot of reading up to do I've noticed, been getting my head around the method of having a fast clock timer/counter, and setting up a compare threshold to put out a pulse once the threshold is reached. Much higher frequencies can be reached this way. I hoping to find way to reset the timer.
Tutorial here:
http://www.evilmadscientist.com/article.php/avrdac
Another route I've come across is playing samples, as in Sebastian Tomczak's sketches (ArduinoBeats, ArduinoDrumMachine)
I haven't quite understood yet what's happening there as well, but I imagine an eight bit waveform sample could be played looped? And then have options like dial in a waveform with analogIn, generate some random bytes or make some presets like sinus, triangle, sawUp, sawDown, some blockwaves with different pulswidth. You can't morph between two bytes can you? I guess not, no such thing as half a bit . You could have a small array of bytes to alternate, wave-sequence or whatever.
Then you can morph between two waveforms once you have figered out wich byte puts out what, but this seems a mega project. Not my cup of tea for the time being.
I've got some 4051 (de)multiplexers now, need to make a board with these and some R2R DA converters, 386 amp and small speaker. Someone might design a shield for this. Seems a usefull addition to me.
The mind overflows with options, but too often I get reminded of my electronics an programming incompetence, so spend lots of time reading stuff on arduino.cc lately.
if you haven't seen it already, check out the APP shield from critter & guitari -
http://www.critterandguitari.com/home/store/arduino-piano.php ,
the code is a bit hard to follow if you don't speak AVR/C, but it's makes some great sounds
I've been checking out your site and flicker stuff. Nice. You've been very productive.