First thing to do is try the debugging steps to make sure it isn't something simple. Asurion is known for their repair centers and handles Google phones and they have a page dedicated to the subject of non-functional Pixel microphones.
Google Pixel microphone not working? Here's how to fix
I'd suggest going through their troubleshooting steps, and if you get to the part where it says to bring it in for repair, then read on.
You could also try running the Pixel diagnostics; iFixit has a copy of the manual for that software right here.
Pixel Diagnostic Tool User Manual - iFixit
The big problem you've got is that the bottom microphone, the one you use during a phone call, is mounted directly on the motherboard. As such, there isn't a part you can just remove and replace like, for instance, the top microphone. Here's the microphone you'd need to replace.
Of course, to even get to it you'll need to take the motherboard out of the phone.
Google Pixel 5 Motherboard Replacement - iFixit Repair Guide
To be honest, I'm not even sure how those microphones come off the circuit board; I'm guessing you may need a hot air station, but I could be wrong. I believe this part is what you'd need to be able to replace that microphone.
Microphone for Google Pixel 5 by Maxbhi.com
Unfortunately, I'm not sure where to go if you don't happen to live in India, though. You might be able to purchase the upper microphone and remove it from the flex cable it comes on; chances are good it's the same part, but no guarantees.
At this point it's sounding like this may be more trouble than it's worth to try and fix; entirely up to you, of course, but I'm afraid it's not a simple fix even discounting the hazard of removing that delicate screen.
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Google still provides repair services for the Pixel 5!
Visit https://store.google.com/repair, enter your IMEI and choose 'Audio Problems' since there is no microphone option.
by Pixel