I found a strange pattern in the last 4 of 5 musicals I have seen.
High Spirits The Ghost Brothers of Darkland County Titanic The Secret Garden The Secret Garden (different production)
All but, Titanic have singing ghosts (and you could argue that all characters in Titanic are singing ghosts).
So how may other shows have singing ghosts?
I have thought of a few... Ghost Les Miserables Addams Family Lavender Girl The Lawnchair Man A Christmas Carol
Others that are questionable: Follies (do the past selves actually qualify as ghosts?) Fiddler on the Roof (does what Grandma Zietel do qualify as singing?) Death Takes a Holiday (are the characters ghosts or saved from death?)
Into the Woods kept coming to mind, I thought it was because I saw the same lead play the ghost of christmas future in A Christmas Carol and the Witch in Into the Woods. But, I can't remember the specifics of the singing ghost in Into the Woods.
Cinderella's Mother is a ghost and has several small singing moments throughout the show...I believe the role is often played by the actress playing Rapunzel, but I may be wrong.
Grandma Tzeitl isn't the only singing ghost in Fiddler, there's also Fruma Sarah. Of course they only appear in a dream, and a fabricated one at that, so I'm not sure if they truly qualify as ghosts.
The Baker's Wife and, perhaps also Jack's mother, appear briefly after they are dead, no?
Depending on how the scene is portrayed, in Urinetown, Bobby's part of Tell Her I Love Her could be a ghost singing, or just Little Sally remembering what he told her.
Sarah and Coalhouse's ghosts at the end of Ragtime {"Wheels of a Dream (Reprise}".
ETA: Eva at the end of "Oh What a Circus." The libretto says: "A non-descript GIRL moves through the pageantry of the funeral. She sings as the voice of the dead Evita."
Salve, Regina, Mater misericordiae
Vita, dulcedo, et spes nostra
Salve, Salve Regina
Ad te clamamus exsules filii Eva
Ad te suspiramus, gementes et flentes
O clemens O pia
Everything Gabe says in Next to Normal - though he could be considered a figment of Diana's imagination instead of a ghost.
Does "Mama Look Sharp" in 1776 count? The boy comes onstage and talks to the people in the room (janitors, maybe?), but then he sings that song in first person and it's just about him.
Spring Awakening Book of Mormon (if you count the people in the Hell Dream?) Billy Elliot (the mom) Little Shop of Horrors (at the end) Assassins (Lee Harvey Oswald scene) The Story of my Life Sunday in the Park with George (depending on how you interpret that last scene)
I don't consider the characters in Follies or in Grey Gardens to be ghosts. I consider them to be shadows of the past, for want of a better term. I consider a ghost to be the spirit of a deceased person which is, for some reason, stuck in this world. With the exception of Joe Kennedy and Mr Beale, none of those characters are deceased...a big difference in my mind.
Art has a double face, of expression and illusion.