Open the Bluetooth speaker, get to the speaker drivers inside and check for any tear within the diaphrams. If any tear is found, seal it off with a THIN layer of glue. Personally, I use the white glue used in gluing furniture and book binding. It will take quite a long time, say four hours, before it dries. Note, apply a thin layer of glue over the tear so as to allow the diaphrams to bounce freely outwards and inwards. This will ensure that the sound quality is not compromised by not ending up sounding HIGH PITCHED. This fix of patching up the diaphrams with a thin layer of glue will work for most, if not all, Bluetooth speakers. Cheers!

Getting another replacement daughter board should fix the problem because some replacement board has faulty mic from factory.

Just get another daughter board. Some of the new replacement board has bad mic from factory.

This has happened to me too several times. Even after changing the mic, the mic did not work only picking up noisy sounds. I was fed up and had to get a new daughter board which solved the problem.

Open the device. Try disconnecting and reconnecting the Body Control Module (BCM) connectors. Worked for me!