(Disclaimer: I don't own My Little Pony.)
It was a good day for Equestria. The princesses had defeated Discord, put him in stone. All of the ponies celebrated that, and cheered for the princesses. But the princesses themselves, were worried about something. They'd kept a happy face on for the other ponies, and it wasn't a total lie, they were glad they'd succeeded, but as the party disappated Celestia and Luna allowed themselves to look at each other in worry.
"This is bad," Celestia said.
Luna nodded. "Agreed."
About a week ago, Discord had gotten a new idea for causing chaos. He used some sort of magic that not only allowed ponies to but into song without any prior plan, but encouraged it. Since they'd already been planning to take on Discord, the princesses had assumed turning him to stone would end the magic, but the celebration had been done in almost all song. The only one who knew how to reverse it was Discord, and even if he wasn't stone they doubted he was going to share.
"Sister, why do you think no one else noticed?" Luna asked. "Surely they should think it strange they're singing."
Celestia looked at the sky thoughtfully
"Well . . . there are only three alicorns in Equestria that I know of, maybe he didn't bother making the magic strong enough to completely effect us," Celestia suggested.
"But it is effecting us!" Luna cried. "Like right now . . . I think we're talking in rhythm."
Celestia's eyes widened as she realized Luna was right. From the way they were talking, and moving, another music number was coming up.
"DISCORD!" Both the princesses yelled.
Discord meanwhile, had to admit, while being in stone sucked, songs carried, and this was going to make things slightly more interesting. Despite being immune to his own magic maybe he'd sing too, when he had time, when he wasn't stuck in stone.
(I always thought there must be some weird magic in places like Equestria where characters can burst into song and it works. Don't know why I thought of Discord being behind it, but hey, why not?)