Microphone Problem

Watch Ultra shows warnings out of nowhere for "loud environment +100db" while it is completely silent.

After this happens, Siri does not understand commands anymore, if you try to record a voice memo no speech is to be heard in the recording but only cracking sounds.

Restart / unpairing did solve the problem for the first 3 times, but failure returned every few days.

4th time the problem stayed even after unpairing and restarting it.

Shutting down the unpaired watch for 24hrs did solve the problem again but failure returned 1 day later.

Watch was sent to repair: Apple does not find the failure...

Watch is only 3 weeks old, problem with microphone occurred after the 3rd day using it.

(Watch has no damages, no contact to water, not falling to the floor, Watch OS and IOS on the latest versions)

Somebody has an idea what to do?

The watch is updated to the latest version 9.4.2, I have already sent the device for assistance but the problem has not been solved. Can you explain me how to solve?