Board Thread:Spoilers!/@comment-27060727-20170222030619/@comment-27060727-20170424042552

"Stylistically, each song is suited to the character’s journey, so that when we see Hook singing, we can say that song does take place in a flashback, and it fits tonally in what a pirate would be thinking and what Hook would be thinking and therefore singing at the time we see him, which is kind of a pirate with a dark streak. In the same respect, when we see Snow and Charming’s song, there’s a duet and a true love element to it. Through all of them, we try to have the songs and the style of the songs tie into who the characters are and what they’re going through. The composers did an incredible job. These songs surpassed our expectations when we heard them, and then when we heard our cast and what they could do with them, it surpassed even further. So we’re really excited about it."

http://ew.com/tv/2017/04/23/once-upon-time-musical-wedding-spoilers/

So Hook's song IS a flashback. And they might have confirmed a Snow & Charming duet there.