Can I choose when Siri speaks on my Apple Watch? If I’m in a loud or quiet place can I choose whether Siri on my watch speaks aloud?
Yes in settings on the watch:


Tap Settings > General > Siri.
Choose: Always On, Control with Silent Mode, or Headphones Only.


Apple also points out your voice needs to download over WiFi to the watch when you select or change it on your iPhone. 


https://support.apple.com/en-us/HT205184