Hello ArtHendrickson,
Welcome to Apple Support Communities.
We understand that you want to use Shortcuts to change Siri’s volume. We are happy to help.
While there is not a Shortcut in the gallery to change Siri’s volume, you can create your own custom shortcut using the steps here: Create a custom shortcut on iPhone or iPad - Apple Support
"Create a custom shortcut on iPhone or iPad
It’s easy to create a new, custom shortcut using the Shortcuts app for iPhone or iPad:
- Step 1: Create a new shortcut in your shortcuts collection.
- Step 2: Add actions in the shortcut editor.
- Step 3: Run your new shortcut to test it."
Let us know if you have further questions.
Take care.