We love your Adam Sandler movies. We’re here to coerce Earthlings to make many more of those. We want more Paulie Shore movies, too.
As long as they don’t couch their request in the form of a 1980’s video game.
Aliens might want more Jerry Lewis movies, too.
And Billy Jack.
Nanu, Nanu! Shazbot!
No message, they just begin fumigating.
or…
“We apologize but we don’t have the time or capacity to save your pets or domestic animals, or any of the other non-silicon-based life forms on your planet.”
“This is a Test. It is Only a Test of the Galactic Warning System. If this had been an actual emergency, you would have been directed to contact your Galactic Overlords.”
“The following systems are closed today due to ion storms.”
“Don’t worry. The fire’s out!”
“You’re gonna love this invasion. I’ve always had a great relationship with the humans.”
“I am Dyslexia of Borg. Your ass will be laminated.”
Yahoo Serious.
Galactus hungers!
People of Earth, answer these questions:
Which one of you said “Take your stinking paws off me, you damn dirty ape!" to my Mama???
Who is this Willis and what’s he talkin’ bout?
By the way, which one’s Pink? You all look alike to us.
Paging Mr Green, Mr Soylent Green.
We wish to study your reproduction methods. Prepare to transport to us this Hal Briston and his sheep - oh ewe, we kid 
In a voice eerily similar to The Big Bang Theory’s Penny, the audio message is “We have been monitoring your Cecil Adams from the 23rd Century, and we have determined that he is now ready to join us. His unique genius is our best hope for bringing peace to a vast and troubled galaxy. Plus we need somebody to write our The Universal Straight Dope column.”
First Contact will take the form of an indecipherable post in MPSIMS from ZRPAMT23, a newly-joined member. Thinking it’s one of those bizarre relationship initial posts, the usual subjects savage ZRPAMT23. Just as the Mods are ready to drop the ban hammer for trolling and lock the thread, a message appears: This is my final communication. Don’t make ZRPAMT23 angry … you wouldn’t like ZRPAMT23 when I’m angry.
If you’d been nice, ZRPAMT23 was going to fix that 120 second delay between searches AND tell you what 14 k of g in a f p d really means. Instead, ZRPAMT23 is going to put all your planes on treadmills, declaw your cats and circumcise or uncircumcise you, depending.
And 42 is NOT the answer to life, the universe and everything.
“Greetings, Earth dwellers. We’ve noticed your planet will be dead in 1,500 days and so have made preparations for your eternity. On close and thorough inspection we’ve seen you worship your chosen God every year and, so, have plans for you to spend eternity in shopping malls, listening to Christmas Carols. Congratulations!”
“We are your feline overloads… Take us to their palace where your fate will be decided depending upon how our Earth cousins have thrived.”
"Greetings Earth Creatures, you are about to enjoy that which is known as “The Invasion of Earth!”, Kraang has need of this planet for Kraang to live on. All inferior Earth life forms will be mutated into superior Kraang life forms by Kraang!
“We’ve figured out a solution to your longevity problem; the treatment has been dispersed into your atmosphere and should enable you to complete your lives in a quarter of the customary time.”
“Where da white women at??”
apologies to Mel Brooks…
“Just turn your head to the left and cough…”
Most of those are frightening, but I suspect that there is one reply, that if it was all there was, that scientists would probably start jumping out of buildings due to the crushing disappointment:
“We received your golden record. LOL.”
“Pawn to second queen’s third rook two.”
It’s true, we are aliens! But what are you going to do about it? It’s a two-party system. You have to vote for one of us!
Well, THAT was the last straw!