A working audio beat detect circuit for a microcontroller using a microphone input
This circuit uses a digital pot to allow the microcontoller to automatically adjust the gain of the circuit to suit the ambient audio volume, but you could replace this with a standard pot if this isn't needed. The low pass filter is a copy of Graham Mitchell's nifty solution from this page:
http://digital-diy.com/forum/electronics-projects/sound-to-light-beat-detection-t1220-10.html
The circuit works well and allows the microcontroller to detect beats by looking for filtered atod reading peeks of a similar amplitude to the pre-filtered atod reading peeks. The following are some captures taken from the 2 test points using a signal generator feeding into an amplified speaker:
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