Martin documents his experimentation with Arduino and realtime audio signal processing and generation –

The first set of examples alter an incoming audio signal and put it back to an audio output. We achieve effects like Reverb, Phasor, Flanger or Ringmodulator. The second set of examples are outputting computed waveforms like Sinewave, Bell and Xylophone sounds.

Along with a helpful tour through the often confusing world of timer interrupts, source code is available for all the tasty effects mentioned above. This is an excellent starting point for those looking to delve deeper into Arduino audio. – Arduino Realtime Audio Processing