I'm running Linux on the MBA. I can't make the microphone work.
I test using alsamixer and
arecord -Dhw -fdat -vv mic.wav together with
aplay -Dhw -vv mic.wav
It seems there is a problem with Linux kernel trying to decipher Apple corrupted 'BIOS', in particular the mic pins are not seen/wrongly assigned.
As far as we are concerned here, is there any difference between using rEFIt and BIOS-compatibility mode (see https://wiki.archlinux.org/index.php/MacBook where they claim to have everything working out of the box on mba1,1) and between, say, Ubuntu and Arch?
A nice proposal would be to compile the latest Linux kernel and run hda-jack-retask or a similar patching program, to find the right configuration (in the case of Realtek codec, the missing things I'm supposed to check are either some vendor-specific COEF verbs, EAPD or GPIO setup.), and then come up with a kernel patch to address the issue.
The MAIN PROBLEM here seems the lack of data on how Apple implements and controls the codec, namely whether they have the mic pins as virtual pins, or something else.
Some useful data:
The output from alsa script run as root http://www.alsa-project.org/db/?f=adae8ebee1007043fe83414ac4972319e02255fa
The command hda-jack-sense-test -a (with HP connnected)
Pin 0x14 (Internal Speaker): present = No Pin 0x15 (Green HP Out): present = Yes Pin 0x16 (Not connected): present = No Pin 0x17 (Not connected): present = No Pin 0x18 (Not connected): present = No Pin 0x19 (Not connected): present = No Pin 0x1a (Not connected): present = No Pin 0x1b (Not connected): present = No Pin 0x1c (Not connected): present = No Pin 0x1d (Not connected): present = No Pin 0x1e (Not connected): present = No Pin 0x1f (Not connected): present = No
Most likely the chip is Realtek ALC885 (compare also ALC889A) http://guide-images.ifixit.net/igi/bBTSqaeK5JpQ1AWe.large , although at the moment alsa reads it as ALC889A
Takashi Iwai's tutorial https://www.kernel.org/doc/Documentation/sound/alsa/HD-Audio.txt
Some people researched the original files from a running OS X installation on this same model (I think the relevant files are AppleHDA.kext/Contents/MacOS/AppleHDA AppleHDA.kext/Contents/PlugIns/AppleHDAHardwareConfigDriver.kext/Contents/Info.plist AppleHDA.kext/Contents/Resources/layout12.xml.zlib AppleHDA.kext/Contents/Resources/Platforms.xml.zlib) http://www.insanelymac.com/forum/topic/220090-alc889a-pin-configuration/#entry1554954 , even though there seems to be a mismatch with what OS X sees as the mic pins and what happens under Linux (Here help would be mostly required)
Datasheet http://www.realtek.info/pdf/ALC885_1-1.pdf (from the same Realtek, one can also try to download Linux driver, but this is just taken from ALSA project, as stated in the readme file.)
Here for the first time support to audio (except mic) for MBA2,1 (which is morally the same as 1,1) is patched into the kernel http://www.alsa-project.org/pipermail/alsa-devel/2010-February/025511.html
This guys might have done a similar job, though to a more recent version and for sound globally, not just mic: http://blogs.aerys.in/jeanmarc-leroux/2013/09/15/fixing-2013-macbook-air-ubuntu-sound-issue/ and https://bugzilla.kernel.org/show_bug.cgi?id=60811
Note: Mic's the only thing that doesn't work for the MBA1,1 under Linux, while the remaining part of audio works just fine.