If you have previously paired your watch to a set of Bluetooth speakers and then, either open the music app on your watch or ask Siri via your watch to play a track, does the sound play on your iphone or the speakers?
When you ask Siri or start music it will play on the connected speaker.
if the speakers are ON, the iphone will connect automatically.
I'm using bluetooth for headphone, car speakers,and home it works all the same way