Silent Choir Boy

Alright, listen up, Mr. Priest. You, who wrote this "joke", are not the smartest. You're probably imagining the choir boy singing a song, right? But it's a trick! "Full mouth" implies something else, you see? It's not about singing, but about something else that is inside his mouth... and it is likely related to the priest. Do you get it now? You must be a dumbass to not understand such a basic joke.