You know, my friend, up there in heaven there's no difference between singing, humming and playing an instrument, it is all the same, though it's also different, but you don't need to care about intonation, it will just be there, you won't need to care about tuning, it will tune itself as soon as you think/sing/play.
So Gould, you know, with all that heavenly harmony in himself, from the higher spheres, did just not care if he has a tuned voice or whatever. He had this need to express it all, like he was still up there in heaven. He heard it all so clearly within himself. It's not important if it's in tune or not. He's not a vocalist.
Listen to his music.