Apple Watch 6 Siri not working or sound when I get calls

How about the matching Siri settings - have you got them all active? See here for details:

Use Siri on your Apple Watch – Apple Support

If everything is already enabled, sometimes it helps to toggle the option OFF and then back ON.

Well yes, it's pretty smple, just tap the Voice Memos app on the watch and try recording one:

Record and play voice memos on Apple Watch - Apple Support

They are also automatically synced to your other devices for playback. So if voice memos are not working either then that would be a strong indication that your mic has a hardware failure.

How about summoning Siri with the digital crown, is that still working? And is it functional on your iPhone ?

Could it be that the microphone is defective? Are you able to record voice memos?