I've experienced what Mac2me has described.
After rinsing off my watch under the tap, the speaker was so quiet I could only just hear it when I put it against my ear.
After a night on the charger, the speaker must of 'dried off' and was back to full volume in the morning.
I can repeat this quite consistently, so not worried about it being a defect, more of a side effect of water and a tiny speaker.
OP, do you have the abilty to make/receive a call?