Idea #1
Temporarily "acquire" your fighter buddy's longsword and cast Magic Mouth on it. Each time it scores a melee attack hit, it shouts, "Take that, you scoundrel!"

Idea #2
Enchant that annoying bureaucrat's pen to make embarrassing moaning sounds every time he writes with it in public. 

Learn thaumaturgy so your magic mouth recordings can be 3 times as loud.

Get the actor feat and expertise in deception to make messages in other people's voices. (You can also just alter your voice with Alter Self.)

Haunt someone with an object that only speaks when they leave the room, at dusk, being the last person in the room.

Other ideas here: https://www.reddit.com/r/dndnext/comments/4bdypc/what_are_some_uses_for_magic_mouth/

And here: https://www.reddit.com/r/3d6/comments/9uc0a2/5e_magic_mouth_what_are_the_most_ridiculous/

How would this work with the Malleable Illusion ability for the Illusion wizard. It's duration can be longer than one minute, so can you change what it says at will, as long as you can see it?

Quote from Snakerat16 >>

How would this work with the Malleable Illusion ability for the Illusion wizard. It's duration can be longer than one minute, so can you change what it says at will, as long as you can see it?

 Yep.

Partner up with a jeweler. Magic Mouth rings that tell their wearer how pretty they are when they put the rings on. Sell them as Rings of Self-Confidence. :P 

Mischief of the day: Using Magic Mouth to prepare lines that you want to be able to say in a Zone of Truth

Detect Invisibility? 
Condition: invisible enemy within 30 feet/sound of footfall from unseen creatures within 30 feet.
Triggered response:  "invisible enemy within 30 feet"
Alternate triggered response: An arrow pointing towards the target

Evasive maneuvers!:
Condition: a weapon within 30 feet is aimed at you.
Triggered response: "you're being targeted within 30 feet"

Spectral Secretary:
Condition: "specific target approaches you"
Triggered response: whispers a message to you. The message you record details the person's name and notes possibly why and where you met this individual.

Hype-man: Cast on stone(s)
Condition: certain hand motion made by yourself
Triggered response: Your Theme and personal introduction "think WWF" wrestler intros.

Kenku Cheat mode:
Requirements: a Friend/ally/paid NPC  
Condition: when held by you and you make a Kenku chirp
Triggered response: stone speak as many words they can come up with in a non-rushed manner recorded in pebbles allowing you to "speak". to be renewed daily to have the vocabulary on tap.

poopy poopy stinky butt? ah, XPTL3 jokes.

I might learn this spell just to cast it in his home lol

A man walks into the Kobold Wizard's lair, he begins scouting for books and objects. Suddenly, from the painting above him is a shout. 
"You got Ligma."
The man responds "What's Ligma?"
Then suddenly, the portrait at the end of the hall shouts at absolute full volume "LIGMA BALLS"
And the two portraits start laughing.

Could you cast this on a Bag of Holding and have it trigger when it is out of sight of any party member and shout "HELP!  I am being stolen!"?  The spell says the object mustn't be worn or carried when cast.  But after the casting, can the mouth be moved or does it have to remain static?  This spell reminded me of a magic bag in 1e or 2e that would yell when stolen.  If the Magic Mouth could move with us, then we could trap our belongings.  But, also that could cause problems too if our stuff is dropped and we are trying to be quiet.  And would probably have another trigger to shut if off if needed.

Quote from Giffard >>

I realized that since this spell can be triggered multiple times until it's dispelled you could totally make walls have a conversation with its self any time someone enters a hallway.

"Hey, what're THEY doing here?"

"Don't know but they're UUUUGGGGGLLLLLLYYY,"

and so on.

 

 this.

QUESTION???????
Me and a fellow play disagree on this spell. Please help me and give me your takes. Thank you.
He cast it on an object he has in his possession so that "nobody can take an object from him except himself, otherwise it goes off".
He argues it's magical and works. I argue if it doesn't see/hear, the item being stolen how would it know?
Such as by a person with a high sleight of hand. In older editions it specifically didn't effect invisible creatures.
Please explain and help. Thank you 

Make a ton of magic mouths and have each of them say something when the other mouth says something so it's just an infinite loop

Since there is no oxford comma and the context of the rest of the spell I am inclined to say it is not an exclusive "or".

Could you get ~34 buttons, for each letter pronunciation in English, and make a “keyboard” that made each sound when touched?

Going to use this spell to Rick role fools in D&D

childrens’ toy sword: when it is swung, dad’s voice makes sound effects! “Swoosh, clang, eeeeyahh!”

Part of a multi spell set: magic mouth on a mask that screams when minor illusion is cast on it to look like a ghostly face. The mask is worn by unseen servant who is also wearing a long cloak. Using dancing lights under the cloak to create a ghostly humanoid form…

Imagine casting multiple magic mouthes to scream as anything comes within 30ft. Dense enough and surrounding a point of interest would make for a creepy defence system.

Quote from Vonkohorn >>

You would not be able to get multiple castings of a spell or cantrip just by recording the verbal component (and keep in mind that Shatter also has Material and Somatic components).  Accessing the Weave requires intent and manipulation from a caster, otherwise we'd sometimes get random spell castings just from the wind howling in just the right manner.

 I kinda like the idea of wind casting spells.