Hi Tim,
As of KM 8 you can indeed have a macro be triggered when the audio output changes, so this should definitely be possible. I think the easiest way to do this may be by first installing SwitchAudioSource; I’ve used this myself in a macro that automatically restores my Mac’s audio output to the built-in option when plugging in a DualShock 4, which also grabs the audio by default even though I don’t want it to. I’d suggest taking a look at this thread, especially the post I linked to, and see if you can’t figure out how to make it all work for your own ends. If you get stuck, feel free to post again and we can offer more specific help.