From Nookipedia, the Animal Crossing wiki
Villagers will sing their song to the player. They will try to impress the player with their tune, so the player may go to every animal and find the tune that strikes the right note. Tunes that the player would like to be made the new town tune must be responded to with "You're hired!", and the tunes that the player does not want must be responded to with "That won't do". This may cause the town tune to change multiple times throughout the day, but only the last tune that the player hires will be kept.