Bill: "They don't know about us"? What's that?
Tad: I say we go find out.
Centy: TO THE INTERNET!
(I'm just assuming this is the song you're talking about.)

[There should be a GIF or video here. Update the app now to see it.]

(After video)
All: 0_______________________________________________________________________________0
Bill: 0\\\\\\\\\\\\\0
Tad:...So the dare is for Bill to sing this to Mabel?
Centy: THAT'S A WONDERFUL IDEA.
Bill: NO!
Centy: Oh, yes you are.
Tad: *Grabs Bill* CMON, BILL! LET'S GO!
Bill: NONONONONONONONO.
Centy: YESYESYESYESYESYESYES.
*A few hours of kicking and arguing later*

*Meanwhile, at Mystery Shack*
Mabel: *Coughing up several erasers* Well....That concludes Mabel's guide to having 2 tongues. Turns out erasers aren't exactly the best tongues out there. WELL! BAI!!!!! *Turns off camera* Well, that was the weirdest moment of my life.
*Piano music, Bill rises from the floor with some blue flames with a piano like from the one moment from Weirdmageddon part 3*
Mabel:....Scratch that. This might be the weirdest moment of my life.
Bill (Yes, he still has his stupid little high pitched voice): People say we shouldn't be together.
We're too young to know about forever.
But I say they don't know what they're talk talk talkin' about.
Mini Bill from Bill's hat: Talk Talk Talkin' about...
Mabel: Urgh. Bill, if this is another plan to forget about that time you had your sister trap me in a cage, I'm not going through with it.
Bill: Cause this love is only getting stronger,
So I don't wanna wait any longer.
I just wanna tell the would that you're mine, girlllll....
Mabel: Nah, Nien, no, no no no. I'm not going through with this. *Turns back, crosses arms*
Bill: Oh, they don't know about the things we do.
They don't know about the "I love you"s.
But I bet you if they only knew,
They would just be jealous of us.
Mabel: NOPE! *Covers ears* LALALALALALA! I'M NOT LISTENING!
Bill: They don't know about the "Up all night"s.
They don't know I've waited all my life.
Just to fall in love, it feels this right.
Little Bill: Baby they don't know about-
Bill: They don't know about us.
Mabel: *Starts shaking* Not...Gonna...Sing....
Bill: Just one touch, and I was a believer.
Every kiss, it gets a little sweeter.
It's getting better,
Keeps getting better all the time, girl...
Mabel: GAH! WHATEVER! *Starts singing with Bill*
Mabel and Bill: They don't know about the things we do!
They don't know about the "I love you"s!
But I bet you if they only knew,
They would just be jealous of us...!
They don't know about the "Up all night"s!
They don't know I've waited all my life
Just to fall in love, it feels this right!  *Mabel grabs a Mabel puppet out of nowhere*
Little Bill and Mabel Puppet: Baby, they don't know about-
Bill: They don't know about- *Gets kissed by Mabel, turns dark red* MMPH?!
Mabel: *Lets go after a few minutes of kissing, slightly blushing* They don't know how special you are.
They don't know what you've done to my heart.
They can say anything they want
Cause they don't know about us.
*Opens door* See ya real soon. And remember, Karaoke isn't about sounding good. It's about sounding terrible TOGETHER... And also remember, THEFUTUREISTHEPAST,I'MLEGALIZINGEVERYTHING,BUYSMILEDIP,BYYEEEE! *Goes through door, closes door*

*Back at Bill's house*
Bill: *Walks in blushing like a maniac*
Centy: BILL! HOW DID IT GO!? PLEASE TELL ME YOU KISSED!
Bill:.....I learned that karaoke is about sounding terrible together, that's for sure.