MobileMammal, baby, here are your singing trolls and kids. Kinda.
Um, what's happening here is...um... karaoke night? Dave and Karkat appear to be singing something by troll Usher. Now, Alternian music is delightful...but some humans find it strange and grating on the ears. Namely, Rose and Jade.
Gamzee's sportsbra is an inside joke, don't even try to get it.