Series 6 Watch, No Sound.
My Apple Watch Series 6 suddenly doesn’t stop making any sounds, just vibrations. I noticed that when I hit the watch, the sound comes back, but it goes again after some time.
Apple Watch Series 6, watchOS 9
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Percussive Maintenance does not sound good.
Start by swiping up to open Control Center and check settings like Theatre Mode, Silent Mode, because sometimes they get set by accidental taps during sleep. If they are OK check settings in your Watch App (or on the watch itself) under General/Sounds & Haptics.
The final check would be to test the speaker itself. The easy test is simply to ask Siri a question which should have a spoken reply, "Hey Siri, what is the time?"
If all these confirm a problem you probably need to get the watch examined and serviced by Apple's specialists. Contact Apple Support to make the arrangement