I own a late 2013 iMac 27". The following setup worked for months until yesterday:
The 3.5mm port is connected to a Y-Adapter, the adapter let me use two headsets at once. I use my headset and my stereo (which as new mic and only is for output) on the ends. In-between headset and y-adapter I use a 5 meter extension cable. That worked very fine the last few months.
Yesterday my friends reported my mic sound is bad and they hear my keyboard while talking. I checked and saw that the mic from my headset was no longer recognized within the audio settings. Sound itself works fine on the headset, as well as on my stereo. I rebooted, nothing changed.
The (bad) solution was plugging in my headset directly to the build in 3.5mm port. External microphone shows up in the settings. Testing the long extension cable and the Y-adapter I came to the conclusion both work fine. It is very unlikely that on both cables the mic channel broke at the same time.
I tried my (never touched before) headsets from my iPhone and my HTC, both only work when directly plugged in, but not over one or both cables.
Directly plugged in headset sound far worse then over the previous setup. According to my friends, which hear me everyday via TeamSpeak.
The only thing I did with audio was to have a Skype session in the night (I usually don't use Skype). I read now that skype + OS X sometimes lead to missing microphones, but only within the Skype App, not affecting the system input. So I assume that this is not related to my issue.
The setup is sadly pretty unique.
Anyone experienced similar issues or have any suggestion on how I can fix?