I decided to try this solution, and didn’t find it entirely straightforward, either. Here are directions for iOS 15. Hopefully it’s similar for earlier versions.
- Locate and open the Shortcuts app.
- If you don’t see an Automation choice, but you do see < Shortcuts at the top of the screen, click it.
- Click Automation.
- Click Create Personal Automation.
- Scroll down the New Automation screen until you find App, and click it.
- Click Choose, next to App.
- Find and select Roon.
- Click Done at the top.
- Select the radio button for Is Closed.
- Deselect the radio button for Is Opened.
- Click Next at the top.
- Click Add Action.
- In the search bar, type: Volume.
- Select Set Volume.
- Adjust Set volume to to the appropriate %.
- Optionally turn off Ask Before Running.
- Click Done at top.
Hope this helps others.