MacBook Pro Microphone issue troubleshooting

Dear community,

I have bought a used MBP A1707 (15 inch 2017 Model) a while ago, which is working fine - except the microphone.

The internal microphone is completely dead and my external headphones' microphone is so muffled and silent that it is no use at all. I verified the external mic (uses the 3,5mm audio jack) to be working fine on another computer.

A complete re-install (delete disk partition, re-install from wifi) has not changed anything. NVRAM reset has been done without any help. I therefore tend to believe that my issue is hardware related.

Before taking this beauty apart, I would like to know which parts could be causing this issue. If it were just the microphone, the external MIC should still be working fine, shouldn't it?

Any ideas what I can do next to approach the issue?

Thanks in advance

Kind regards

Seb