It’s B&W, probably late 50’s or early 60’s. It seems to take place in the late 40’s.
A handful of men (4 or 5) are in prison, and at least one of them is there unjustly. They have garnered favor with the guards/warden and are given a special, bigger cell. One night, they have a jam session of sorts, where one plays the harmonica, many sing, etc. One of them says “I’ve been singing country music since before they came up with the words ‘country music.’” Then, some big, muscular, oafish character starts singing in a beautiful tenor voice, and all are amazed.