Hey MS_LS,
If you're attempting to adjust Siri volume from your Apple Watch go to Settings > Siri and then tap on the volume down button, instead of sliding to turn the volume down. If you want to use the slider, tap the volume button first and then you can slide.
We hope this clears things up and helps out, but if not, restart both your iPhone and Apple Watch and try again.
All this information and more is available at this link: How to change Siri volume
"Change Siri's volume on your Apple Watch
Raise your wrist and speak into your Apple Watch. Just say "Hey Siri, speak louder" or "Hey Siri, speak quieter."
You can also change the volume in Settings:
- On your Apple Watch, open the Settings app.
- Tap Siri.
- Scroll down to Voice Volume — Speaker.
- Tap the volume controls. Or tap the slider, then turn the Digital Crown.
We hope this Information helps out. Take care!