Board Thread:Spoilers!/@comment-27060727-20170222030619/@comment-2153169-20170430002710

"Simply said: We learn in flashbacks that Snow White, when pregnant with Emma, inadvertently cast a spell that makes highly emotional people only able to express themselves through song. That allows for musical numbers both in the Enchanted Forest of yesteryear as well in present-day Storybrooke, when the spell resurfaces."

http://tvline.com/2017/04/29/once-upon-a-time-season-6-musical-episode-preview/