Unseriously, you want to get him on YouTube, he'd be a star!
Seriously, I can see the problem. Music is everywhere. To a dog, this probably isn't singing, it's howling. How do you react when he 'sings'? Maybe he's hooked on a reaction from his 'pack'? You could try giving him 'time out' at the very moment he starts the howling (taken out of the room, denied any attention, the door closed, and only when he falls quiet, let back into the room again, and made a fuss of, and rewarded) ? Wolves howl for different reasons than dogs. To rally the pack. But mostly dogs howl when they feel weird. It is possible the sound is getting to his ears. What about really gentle, low volume nice classical or romantic music? If you could get him used to this playing in the background, he might break the habit. Don't know.....good luck trying things......
Maybe he has sensitive ears? Something to look into...