First Siri talks at two levels: quiet when you raise to talk and much high level when you press the digital crown in and hold until Siri responds.
On Apple Watch Series 3, you can choose when Siri speaks:
On your Apple Watch, tap Settings > General > Siri.
Choose an option: Always On, Control with Silent Mode, or Headphones Only.
If Siri doesn't speak, your Apple Watch needs to download the voice. The download happens when you’re on Wi-Fi and your Apple Watch is charging. To give the download time to complete, try charging your Apple Watch overnight.
To check the status of the download on your Apple Watch, tap Settings > General > Siri, then scroll to Siri Voice. If the voice needs to download, you'll see this message: "Siri will speak with the same voice selected on your iPhone. Waiting to download. Download will occur when your watch is charging."