@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.