In previous editions a *Magic Mouth* spell could not be used to trigger an object, but in 5e it's not quite as clear.

I'm designing a scenario where a crew of thieves enter a *Mirror of Life Trapping*, which then has a *magic mouth* spell cast on it and is locked in the vault of an art gallery. Their scheme is to have the magic mouth trigger at midnight, speaking the words to release the thieves so they can loot the vault.

In the PHB description for *magic mouth*, it states:

> 
> When that circumstance occurs, a magical mouth appears on the object
>  and recites the message in your voice and at the same volume you
>  spoke.
> 
> 
> 

As far as the DMG, the relevant section in the Mirror of Life Trapping
description says: 

> 
> While within 5 feet of the mirror, you can use an
>  action to speak the name of one creature trapped in it or call out a
>  particular cell by number.
> 
> 
> 

Clearly the magic mouth is not using an action, but outside of combat are actions really "spent" or is it just a measure of time?

Does anyone know of an official 5e ruling on this, or can you make a case either for or against it? It's for possible publication in a DM's Guild adventure, so I don't want to rely on DM fiat if I can help it.