I've heard and noticed the same things. I guess they didn't want to have both brothers sing in that episode, so they chose Cecil. I wouldn't even really call that singing. That was just Cecil singing quietly to himself. Everyone does that, no matter how good or bad their voice sounds.
Bob on the other hand... Bob can SING. I think his best performances were in "Cape Fear" and "The Italian Bob." I literally melt every time I hear him sing Pagliacci.