I've always loved his singing. It was just such a difference to his speaking voice even when he wasn't playing Gomer Pyle
Ed Ames who played Mingo in Daniel Boone (TV series) had a fabulous voice and was a singer and part of the Ames Brothers long before he got into acting. Interestingly he wasn't an American Indian. His parents were Russian Jewish immigrants. After doing TV and movies for a while He returned to singing and had a career making several hit records
George Clooney singing in O Brother Where are Thou? was hilarious
Meryl Streep and Pierce Brosnan singing in Mamma Mia was quite the surprise. It was noted in something about the movie that she never sang in movies but I have an earlier movie of hers where she did sing - and quite well too
Richard Gere In Chicago was impressive