@Rachel_Thorn I don't have my mac on hand right now, but the best bet for me would be to write some AppleScript using Script Editor, save it as a .app into your /Applications folder and then ask Siri to run it.
Should be able to create things like:
`set volume output volume 50`
To set the volume directly to 50%
`set volume output muted not (output muted of (get volume settings))`
To toggle mute on/off etc
These are just simple examples, but AppleScript is pretty flexible and should be able to switch output devices, etc, depending on how complex you want it.
There are plenty of examples on the Googles.