Drove myself nutty trying to get stereo microphone input under slackware on my macbook pro (MBP 5,5).
I made a screencast under OSX using the built in version of quicktime. I recorded screencapture in one process, my voice and face (from the iSight cam) in another process, and merged them together in the movie editor. The lame result is on youtube complete with utter failure at the end. Makes me laugh when I see it, and it is a word of caution to use a script, not go off the top of your head.
Anyway, that’s not the point of this post. My point is getting the macbook pro internal mic to work under Linux. I ran across several tips and howtos on using ffmpeg to record the screen and capture the iSight input, so I wanted to do that rather than boot into OSX. But for some reason, no matter what I did the recorded sound was in mono and only on the left channel. Immediately I unfairly suspected that my Linux software was to blame, but found nothing on the interwebs about such an issue. I recompiled the kernel, downloaded and installed PulseAudio and all its dependencies, and so on, with absolutely no effect. The only thing I could do was make the input sound worse by using arecord directly.
Turns out, there is no such thing as stereo input on the MBP 5,5 — at least, none that I can find. Under OSX, it appears that “stereo” is actually mono piped to two channels. To test this I made a little audio recording using QuickTime and snapped my fingers right and left. No difference in my headset during playback, just a snap right in the middle of my brain. And if you look up the specs on the Apple website, you’ll see “Omnidirectional microphone”
So the only problem with Linux is trying to mix the left channel to both channels.