Microphone issues - no, limited or static input. No Siri, voice notes

Hello

I’m fairly new to all this and recently started a spare parts business. I have a iPhone 12 that had suffered some water damage and overuse of isopropyl 99.99 during repair.

I’ve replaced the screen. The rear camera appears to have some moisture “halos” on the lens but other than that the phone is perfect. I had to do a DFU restore to get it back to normal though.

The only issues I haven’t been able to fix are microphone related.

The top microphone (Siri, FaceTime) is screwed. At first it picked up very faint fuzzy audio but now dead completely. The bottom microphone can sometimes pickup audio clearly but this appears to be positional. IE if the phone moves about a lot, it can just turn to quiet static or nothing. It’s very hard to replicate. During a call (ear proximity) it can just cut out. So the phone needs a headset to use for that, really.

I did try a new earpiece flex and the issue remained. This makes me think it lies elsewhere and that perhaps the original earpiece flex is fine… I first suspected the earpiece microphone was fried. But this suggests maybe not.

Apart from that and the new display, camera issue the phone is perfect.

Any suggestions of what to try next? I have a good amount of knowledge but am quite new. I don’t know about circuitry or microsoldering but very happy to pick this up if it’s a cheap repair. I have an abundance of spare parts too. I could even move this iPhones internals into new housing. But it seems wasteful… the cosmetics are fine too.

There are no hardware error messages.