Here is the next addition to the list of tinkering for me. For an “Instructable”, this author is very specific and has quite a few coding methods. Off first glance, some of their methods looks FastSPI friendly, except their 38kHz sampling rate code that uses interrupts and port manipulation. They even have some beat detection code used to find falling and rising events on a sampled wave form, thoughts anybody?
http://www.instructables.com/id/Arduino-Audio-Input/?ALLSTEPS