Load Orto and Tap the Shortcuts icon on the bottom bar

Tap 'Shortcuts +' title button at the top of the screen

Pick the player / players you want to decrease the Volume on - the speakers will change independent of any groupings

Tap 'Volume Down'

Set the amount you want to decrease the volume by - each time you trigger the Shortcut

Tap 'Okay'

Check the Shortcut description matches what you want to happen - test it with the little play icon at the bottom right. Then tap the 'Add to Siri' button

Amend the 'When I say' field to your choice of phrase. This is what you will tell Siri when you want to trigger the Shortcut. Tap 'Add to Siri' when ready