Grim Adventures Of Billy And Mandy (the) Easter Egg - Sound of Another Show

This is in the episode "Sickly Sweet" where Billy and Grim put the "mask of the beast" on Mandy while she was asleep, to make her be kind to others, or she will be turned into a hideous beast.

A litte after the start of the show, right after Billy is laughing at mandy, because she has the mask on at the bus stop; Grim is sitting in the living room, watching tv, in Mandy's house, you can hear the sounds of the Kids Next Door cartoon in the background, on the tv.