wvmarle:
Assuming you're going to have just one frequency in your signal (or at least one that's very dominant): microphone --> amplifier --> Schmitt trigger --> digital input --> frequency counter.
I guess you meant to follow this hookup guide.
MrMark:
Post #42 in the linked thread has the most recent code that I used.
Thanks.
Without looking at the code, do you also have a Schmitt interrupts in the code?