Take a look into FMOD and OpenAL
- fmod.org
-
en.wikipedia.org/wiki/FMOD
-
connect.creativelabs.com/openal/default.aspx
- en.wikipedia.org/wiki/OpenAL
Being written in c/c++, both of these libraries can easily be linked against standard iPhone code to be compiled against the ARM architecture of the iPhone.
They are both capable of extracting the information you require from the audio stream of the iPhone’s microphone via the APIs provided by Apple.