@derekhuffman disable fullscreen (f11)

Regarding the ‘keyboard and mouse’ part in your post … I use a wireless set. So the rPi is mounted behind the monitor and only has 2 wires on it: power and HDMI. There’s a USB dongle plugged in that’s for the keyboard and mouse which I rarely use. Specially since I log in remotely. The only time I need the mouse or keyboard is if something bad happened and the rPi won’t boot up normally. But other than that, not needed.

@derekhuffman it’s fine to post it here :) You could do this to ways.
Have a module that’s checking a GPIO Pin with a sound sensor, the module could then trigger a relay or turn on the HDMI output. That would work if the monitor does not need to be turned off completely (I would turn it off to save energy). With this approach you would have the information that’s it is turned on available in the MagicMirror system if you don’t need that (you probably don’t), you could also go for a hardware only solution, Google “activate relay if sound is detected” for that.

The best method would be to just use a PIR-Sensor or is their any reason you want sound detection?