Yeah, this was a Mystery Never Solved. There were moments where it seemed as if Radar was the PA Voice Guy. Other moments, not so much. But someone had to spell him on that duty, given that the 4077th had to be ready for patients 24-7.
Sure, someone to spell him, naturally. The problem comes when you hear the voice at a moment when Radar (or later Klinger) is actually in the room where the mike is located.