Board Thread:Spoilers!/@comment-27060727-20170222030619/@comment-27060727-20170429023215

"As for why the denizens of Storybrooke will be breaking into song, Parrilla teases, “Something happens. I don’t think I can really talk about it, but let’s just say it’s not a choice. It kind of comes upon them.”"

http://ew.com/tv/2017/04/28/once-upon-a-time-musical-lana-parrilla-rap/

So we know they'll be singing in flashback-like situations, but also present-day because there's a song during the wedding. I'm not sure what kind of "curse" would result in us seeing both types....