That TRRS socket actually has of odd configuration of 4 pins so that you can use the genuine Apple headphone/microphone all-in-one 4-pin combo that comes with iPods & iPhones etc. Generally they look like this:
Here is your microphone, right? The Sennheiser ME3. Notice there is a locking ring on that plug? Also, notice how there are two little black lines on the end there? That is the insulation separating the 3 conductive contacts aka 3 pins This is a TRS plug: ( - )( + )(⏚).
It comes with the following caveats:
1. The plug on your microphone has a rotating "locking ring", you have probably found that the microphone isn't working, or works only
sporadically depending on the connection. The issue is that locking
ring prevents the non-standard plug from fully engaging when plugging
into your sound card.
2. Please note that all versions of the Sennheiser ME3 are wired differently than usual headset microphones and are not electrically
compatible with many computer sound cards.
First of all, you're probably going to need something like this:
3. Additional Note: This adapter does not change the unusual wiring scheme used by Sennheiser in the manufacture of the ME3. The ME3 is not compatible with all sound cards. If you are having trouble, we recommend you interface your ME3 with an external USB sound adapter, such as the Andrea Pure Audio (MA) adapter, Buddy 7G or the SpeechWare MultiAdapter.
I don't know what's up with the strange wiring scheme. Maybe it's a stereo microphone, maybe it has something to do with their noise cancellation feature, maybe they use some weird shielding in their cable, maybe they're using balanced cables, maybe they just decided to use an odd wiring scheme and call it a proprietary connector, preventing people from using aftermarket accessories, maybe it was an accident. I can only speculate without digging deeper or seeing a schematic or something. If it were me, I would probably cut the plug off and wire it in a traditional manner. Apparently though, these are compatible with the Sennheiser ME3. They are known as The Andrea Pure Audio USB-SA Adapter & The Andrea Pure Audio USB-MA Adapter, respectively.
If by some miracle you have an ordinary connector, or you find a suitable line-level adapter to make things "normal", you're probably still going to want one of these;
1/8" TRRS male to 1/8" TS female input & 1/8" TRS female adapters:
Yes, it looks and sounds weird. The 4-conductor TRRS plug on this cable uses the CTIA standard configuration, which puts the microphone signal on the sleeve (versus being on the second ring in the OMTP standard). The CTIA standard has always been used by Apple and HTC, while older Samsung, Nokia & Sony Ericsson devices (for instance) initially used the OMTP standard.
Plug the 3.5mm (1/8") male TRRS plug into the audio jack on your laptop.
The smaller of the two 3.5mm (1/8") female jack bodies is the TRS stereo headphone output. Plug your standard earphones, headphones, speakers, or other output device into this jack.
The larger of the two 3.5mm (1/8") female jack bodies is the TS mono microphone input. Plug your (hopefully, now) "standard PC style microphone cable" into this jack.
I had the exact same problem on my MBP late 2015 (running Catalina 10.15.7) with a Rode Mic and a TRRS adapter. Everything was perfectly fine and during a live transmission the mic went dead. I had to quickly switch to my AirPods (awful sound compared to a external mic) to complete the transmission.
I tried ALL possible solutions documented on the web and Apple Support pages. MBP would just not recognize it anymore on the Sound properties.
I tried SMC reset, NVRAM reset, upgrade to last patch (I was one patch behind), upgrade to Big Ser (new OS).. nothing.
I decided to reinstall and restore from Time Machine, since I didn't have interest in running Bigsur yet. Voilà!!! Worked!
It seems the reinstall + Time Machine restore fixed something that was corrupted in the OS.
Best Answer
I am in a nearly identical situation as you. I have been manically searching for this answer for the past two days, finding little. But here is what little I've found:
The Bluetooth mic is used during phone calls, facetime, and speech-to-text. It is not used for anything related to recording, as if the software is only ok with using that input if the evidence is destroyed immediately, if no record is kept. Wired headphones that have mics built-in will work for recordings, but for some reason the Bluetooth headsets are treated differently. This is true for my iPad mini and for my Android phone.
The most plausible theory I've heard is that there might be some safety regulation that is responsible. Like the government thinks it would be too easy to eavesdrop on people through their headsets if they didn't make these restrictions. But that is just speculation.
Anyway, I hope I was able to tell you something you didn't already know. I will be anxiously monitoring your post to see if someone who knows more than me shares new information. Good luck!!