I was just reading a 555 datasheet. If you set up the 555 as an astable multivibrator, than the output should oscillate at the frequency set by 2 resistors and a capacitor when Reset is High, and not when Reset is Low.
Thus I suppose if you had a shift register driving the reset lines of eight 555 timers (or their multple-part per package variations), then you could control 8 tones at once. I don't know how much lag that would be, perhaps using SPI to shift out would be needed.
See a data sheet like National Semiconductor, which has a graph to give an idea of values to use, or you can plug the formula into an excel spreadsheet, pick the frequency and a couple values, solve for the others.
Take the 555 outputs, add them together using an op amp, add low pass filter to keep high frequency switching noise out. Pass on to your sound system.
Make one of the parts some kind of tunable part to dial the frequency in. Could likely do that with a digital potentiometer, use the arduino to measure it and make it autotuning!