You can use something like Cast-web-api to cast an mp3 or any text you want to play (via Text To Speech) to your Google Home but it will require an always-on computer to run the node.js project as well as building your event handler routine in WebCore. It’s not terribly complicated but its not a simple as doing this on the Alexa platform.
I’ve setup something very similar. Streams a Star Trek related beep to my Alexa when our presence changes on our phone. Works very well (when my phone updates it’s presence in a reasonable amount of time)
I returned the google home and am waiting for the echo dot. I setup alexa on my phone and the routine for door open/close but i dont see any option to select a local mp3 or where could i upload something to make it work