Last Thursday my wife & I watched the Gray's Anatomy Musical. Seems like many long-running TV shows do a musical episode sooner or later. Often this is a Very Special Episode designed to garner support for one of those TV awards. Which is why I call this Gray's Gimme an Emmie episode.
It didn't work for us. For one thing, the premise was weak, because it was presented, more or less, as one of the characters, badly injured in a car accident, imagining that she and others were singing.
But they broke that conceit by having singing going on outside the injured person's POV (point of view). Making it a collective fantasy, same as in the traditional Hollywood musical. (see the song "Till there was you from...what was it, DamnYankees?).
They used original music which was rock-y rather than Broadway-y...and forgettable. It looked like the actors did their own singing, with lots more singing assigned to the better singners, led by Sara Ramirez, who came to Gray's from a leading singing role in the Broadway musical spoof "Spamalot," in which she excelled. So there was nothing wrong with her voice. It was just what she was given to sing, and how.
When "Scrubs" did its musical, the conceit came from a patient with some brain disease that caused her to hallucinate that she and everyone else was singing, and it stuck to her POV, and the music was better--that episode worked. It also had some humor, while "Gray's" was strictly dramatic--which also failed to utilize Sara Ramirez' formidable musical comedy skills.
Earlier, the "Buffy the Vampire Slayer" musical, also using original music, also mixing humor and drama, was wonderfully successful. It this fantasy show the conceit was a Broadway demon who magicks everyone into singing--and their song reveals what they've been hiding from everyone else. So the episode doesn't just comment on everyone's situation, as "Gray's'" does--it drives the plot forward for the whole season, it doesn't violate POV, it's musically good, and, since one of the characters had summoned the demon for selfish reasons and not realizing the demon was demonically dangerous, so there's also the underlying moral of a cautionary tale about the Law of Unforseen Consequences. "Gray's" stays shallow.
Probably the father of all TV musical episodes is "The Singing Detective" by Dennis Potter, shown on PBS. That was truly a-ma-zing. Look it up (not the crummy American remake, though).