Most people don't care about that, so you get a gold star.
I doubt anyone who uses a song for a skit or walk-on gets permission. Chances are that there isn't going to be a secret IP agent in the audience waiting to jump on the first person who uses a licensed song. It's not worth worrying about.
Besides, if there is someone trying to spring a lawsuit, it'll be against the con, not you. They'd probably be closed down for the anime viewings and pretty much anything fun you see at a convention. While they're at it... Cosplayers arrested left and right, artists in the alley having their wares taken into custody... Copyright's a strange thing.
Add me on Skype to join CosCom chat group
Izaya (Durarara) - Complete
Drossel Kainz (Kuroshitsuji) - 90% (need new wig)
Judal (Magi Labyrinth of Magic) - 15% (shirting)
Sheik (LoZ/SSBB) - 10% (armoring)
summer!Jack Frost (RotG) - 10% (wigging)
AAC Summer Matsuri
Another Anime Convention