Apple watch series 3 microphone stops working after watch OS 6 update
iOS update seems to be related. The phone is an iPhone 11.

iOS 13 - Killed both speaker and microphone (e.g. The Micky watch face stops announcing time; the face time counterparty cannot hear what is spoken to the watch; Siri cannot get user's voice command)

iOS 13.1 - Speaker works again (The Micky watch face can announce the time). Microphone remains nonfunctional.

iOS 13.1.1 - Both speaker and microphone stop working.

At the end I have to take the watch to Apply twice. At second visit they took it back for repair.

In summary: The watch needs to be repaired.