My music teacher in high school had us sing our part on a song if we were not playing it right. Like singing the melody or if the rhythm was off. So that's what I do and most times it works. If not, CALL SOMEONE!!

But I know an organist that claims to be tone deaf when singing, and he gets by fine. I've heard him sing, and yeah it is bad... but again, he gets along just fine.
Be blessed!
Bishop