I'm having the SAME problem. I'm on a nightly release; 11-20150105. However, it does *sometimes* work as normal, but the fix for it now is to unplug and plug back in a couple or more times and then it finally kicks in. What I've noticed is that the volume in the headphones is also a little lower when the microphone isn't working.
Could a different radio or kernel help? I'm currently on 1.15.50.05.29_10.30.50.08U. Kernel is 3.4.82-g0129413. It's really unfortunate because I use a single in-ear headphone and mic when in the car ('hands free'), so this gets used a LOT as I do 160km every day. Other than that, the ROM is absolutely flawless, nothing else seems to be faulty, no random reboots, nothing. It's basically 99.7% complete.
For those having headset/headphone mic not working try the following:
- using a root file explorer (like the included CM file explorer or ES File Explorer), open the /system/build.prop file
- look for the following line: "
persist.audio.handset.mic=digital" and change it to "
persist.audio.handset.mic=analog" (change the bits in red)
It should work. Reason: for whatever cause, when an AOSP ROM is built, this line is included in build.prop. While this may work on majority of the phones, for Ville it seems not. The behavior if this is set to digital is distorted voice heard by other party or no sound at all, interruptions and so on. I had this issue on all AOSP ROMs so now I edit this line "by default" on any ROM after flashing (or you can edit the file in the ROM zip before flashing). It is valid for KK and LP as well
For the low volume issue there are some tweaks made by Darknites and others by me (they are in several threads like the CM12 thread or Candy5 thread). They're made for LP basically but I think the files are the same between KK and LP. Make a backup of the original file anyway.
All is about altering some values in a file called snd_soc_msm_2x located at /system/etc/snd_soc_msm. You cand extract this file from the phone and edit it on your PC with a txt editor like Notepad+. Look for some entries in the "Device section"(s), do not touch other sections. There are some headset, voice, headpone and so on device entries and each has it's own volume set, usually somewhere between 60-70. You can SLIGHTLY (and I MEAN SLIGHTLY) increase these values (try with 1 to 5 maximum) so if something is 60 make it 65. DO NOT increase these values too much as it can damage your headset or bring distorted sound if they cannot handle the volume.
NOTE: volume heard during Viber, Skype and similar calls it's set by altering the entries containing "VOIP" (Voice Over IP). Adjust them same way
Good luck...