I don't know about a musical but there will definitely be some singing in one of the upcoming episodes. Here are the spoilers on who we can expect to be singing!
I loved the Arizona-centric episode of Grey's Anatomy. What's next for her? — Lori
ADAM: Show of jazz hands: Who wants a musical episode of Grey's? Well, you're probably out of luck, but we will see Arizona sing in the coming weeks! Better still, it's a duet, but don't expect Tony winner Sara Ramirez to be her partner in song. Instead, Arizona will trill with one of her male co-stars. I hear the Chief has a rich baritone.
Source: TV Guide