Apple watch started alerting of loud sound level over 100db many times a day, when there were none.

Which Apple Watch are you using?

Did your watch come in contact with water?

Use the Noise app on your Apple Watch - Apple Support

Monitor your environmental noise exposure with Apple Watch - Apple Support

Did you already try to erase the watch and pair it again to your iPhone, setting it up as a new device, instead of using the latest backup?

Unpair and erase your Apple Watch - Apple Support

Do you often fold your arms? I do. I get false notifications after having my arms folded for several minutes and having a conversation with another person. I only get the notifications when my arms are folded. I’ve learned just to ignore them.