Ooh! I love this question! Let’s dive right in, then—
Papyrus would be all over karaoke. The chance to get up in front of everyone and demonstrate how awesome he is? Why, yes, thank you! He’ll take ten. His enthusiasm is catching and he’s only slightly off-key, so everyone usually ends up having a good time.
Sans would be interested in karaoke for his brother’s sake, but not necessarily for his own. With that said, he, Red, and Stretch will absolutely be the obnoxious singing drunks later on in the night. They usually pick something outside of their range, or they’ll play a serious song for laughs, or purposely misread the lyrics “ ‘scuse me while I kiss this guy”-style.
Stretch will not sing by himself…but part of him wants to. He both longs for the attention, but is so afraid of any kind of rejection that he can’t bring himself to sing alone.
Blue really isn’t all that interested in karaoke for himself, but he loves playing backup for the others. He has the most fun getting up there with Papyrus, because they play off each other well, but he’ll pull his brother on stage under the guise of wanting to sing himself. (Blue’s a good bro.)
Edge will attend, but no force on the surface or Underground could get him up there…except Felldyne. She is the one person that can get him relaxed enough (and drunk enough) to actually do something so undignified.
Red will get up there with Sans and Stretch, but if he’s drunk enough (and maudlin enough), he’ll sing something sad and soft. He’s not all that concerned with dignity, either, so he’ll sing with Papyrus or Blue or pretty much anyone that asks. (And he loves seeing Edge relaxed enough to participate.)
Razz will take no part in this nonsense. He’s not even going to go with them…well…maybe once wouldn’t hurt. (They’ll wear him down eventually.)
Aw, Slim. More poor boy. He hates karaoke. Hates the off-key, drunken singing. Hates that he can’t bring himself to sing even with the other older brothers. He doesn’t attend.
Twist…oh, my goodness, Twist. He is very much into anything that puts him in the spotlight. And, actually, he’s not bad. He’s got a low, rich voice, and he picks songs that suit it. He’ll mess around with the other older brothers, though, and he has made it his mission to encourage Edge, Razz, and Stretch to participate.
Blackberry loves karaoke because everyone else is having fun and, particularly, because his brother’s enjoying himself. (He worries about Twist. It’s hard for his brother to connect with people, sometimes, and he’s delighted to see him so happy.) He’ll do anything to make sure the others, Twist in particular, continue to enjoy themselves.
Oh my god I love these. Blue and Edge were adorable, and then the Swapfells just got damn depressing as fuck. But still, I’m here for this!