The low volume on your audio recordings might be because the FP1 has a secondary microphone on the top end as well. So the recording might have just been what the secondary microphone picked up. Try recording again with the secondary microphone held shut with your fingertip.
The microphone sits on the daughterboard which is unfortunately permanently out of stock in Fairphone’s shop. I think you will have a very good chance though if you create a “buy” topic here in the forum’s unofficial market.
If there is a Fairphone Angel nearby, even better – there is a good chance they might have this spare part for you. It is rarely looked for.
Record bass by covering and without plugging the audio holes.
What I do not understand is that before changing the screen it worked fine and I have removed the cables again in case there is a bad contact.
Should there be a bad connection, would I not record or play?
As far as I know - Main microphone (purple circle; image credit Fairphone shop):
As far as I know - Secondary microphone (purple circle; image credit modified from iFixit (license)):
The connection between daughterboard and mainboard is made via the display assembly, so this can also be an issue with the wiring/connectors on the display assembly.