When we were about thirteen, our Sunday School class went caroling. It wasn't my first trip with them, but it was my last. My thirteen year old musician cousin (piano master and superb vocalist), while we were singing "Away in a Manger", turned to me and put her finger over her lips, and said "you're ruining the melody!" So much for "make a joyful noise". It was a harsh, but valuable lesson. I put a lid on my bucket, and learned to enjoy listening to music.
If you have someone who believes he can sing, but can't, hand him a microphone, and record him singing his favorite song. Sit with him in the sanctuary and play his song for him. Ask him what he thinks.