Which one of the spaghetti westerns are musical?
Little Rita nel West. There is also the even more bizarre - as you can see particularly in the second clip - Per un pugno di canzoni, in which the musical element clearly prevails on the Western part:
is Little Rita worth watching? its available on Italian DVD with English option.
Its alright for a laugh, and if you want to see Gordon Mitchell as an Indian .
I would also throw in Black Tigress aka Loa Colt, The Lady was a Sheriff and Per un pugno di canzoni aka A Fistful of Songs all of them are filled with plenty of singing numbers and the last one is definitely a musical.