Being a Pixel 6 Pro owner myself, I've been keeping an eye on the Pixel problems and can offer you some insight into the issue of replacing the lower microphone.
Okay, first off, I'm afraid Google has done us dirty by having the microphone and the USB-C port soldered directly on the motherboard. I'm sure this saves them a few pennies on each unit they manufacture, but when it comes time for repairs, it's a purely evil decision.
A bit of research was required, but I located the microphone itself on the motherboard. Here's a picture with it highlighted, courtesy of the teardown done by the TechInsights website.
[image|3279388]
As you've already surmised, replacement parts for the motherboard itself are few and far between. You could consider scavenging one off of a junk motherboard, although you'll have to find one first. The other thought I had was that the Pixel 6 non-pro model does have a replaceable top microphone which is available for sale. I don't know for sure that it's compatible, but I strongly suspect it would be. Here's an example of an AliExpress seller who has the part for sale.
[link|https://www.aliexpress.com/i/3256805390350846.html|1Pcs Microphone For Google Pixel 6 Top Mic Phone Flex Cable Repair Replacement Parts - AliExpress|new_window=true]
If you're good at soldering and have a hot air station, this may not be too bad of a repair, but otherwise you may have to find a repair shop who can unsolder the parts from the Pixel 6 flex cable and the 6 Pro motherboard then replace the defective one.
The only other choice you've got would be to just replace the whole motherboard; for some reason they're available on AliExpress, although I have no idea how legitimate these actually are.
[link|https://www.aliexpress.us/item/3256805950560282.html|Original Motherboard For Google Pixel 6 7 Pro 5 Unlocked Logic Main Board Circuits Panel Work 128GB 256GB Mainboard Replace - AliExpress|new_window=true]
At $135 USD a pop, I'd sure try the 6 replacement part first, though!
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In either case you're going to have to remove the motherboard. Sorry to say iFixit doesn't have a guide for that particular repair, but the Spanish site Nadie Me Llama Gallina (Nobody Calls Me Chicken) has just what you need.
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[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/google-580/google-pixel-6-pro-1377/placa-base-12464|Manuals / Google Pixel 6 Pro / Motherboard - Nobody Calls Me Chicken|new_window=true]
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Good luck, and let us know how it all turns out for you!
Hi Steven,
Being a Pixel 6 Pro owner myself, I've been keeping an eye on the Pixel problems and can offer you some insight into the issue of replacing the lower microphone.
Okay, first off, I'm afraid Google has done us dirty by having the microphone and the USB-C port soldered directly on the motherboard. I'm sure this saves them a few pennies on each unit they manufacture, but when it comes time for repairs, it's a purely evil decision.
A bit of research was required, but I located the microphone itself on the motherboard. Here's a picture with it highlighted, courtesy of the teardown done by the TechInsights website.
[image|3279388]
As you've already surmised, replacement parts for the motherboard itself are few and far between. You could consider scavenging one off of a junk motherboard, although you'll have to find one first. The other thought I had was that the Pixel 6 non-pro model does have a replaceable top microphone which is available for sale. I don't know for sure that it's compatible, but I strongly suspect it would be. Here's an example of an AliExpress seller who has the part for sale.
[link|https://www.aliexpress.com/i/3256805390350846.html|1Pcs Microphone For Google Pixel 6 Top Mic Phone Flex Cable Repair Replacement Parts - AliExpress|new_window=true]
If you're good at soldering and have a hot air station, this may not be too bad of a repair, but otherwise you may have to find a repair shop who can unsolder the parts from the Pixel 6 flex cable and the 6 Pro motherboard then replace the defective one.
The only other choice you've got would be to just replace the whole motherboard; for some reason they're available on AliExpress, although I have no idea how legitimate these actually are.
[link|https://www.aliexpress.us/item/3256805950560282.html|Original Motherboard For Google Pixel 6 7 Pro 5 Unlocked Logic Main Board Circuits Panel Work 128GB 256GB Mainboard Replace - AliExpress|new_window=true]
At $135 USD a pop, I'd sure try the 6 replacement part first, though!
Good luck, and let us know how it all turns out for you!