"Turning on" can mean "to release a flow of water with a valve." "I turned on the hot water and let it run for a few minutes." (By extension, it also means "to release a flow of electricity with a switch;" thus, "I turned on the computer.")
A "torrent" means "a huge, rapid flow of water." "After the record rainfall, the quiet stream became a raging torrent."
A "gush" means a sudden large flow of a liquid. To me, a "gusher" means, specifically, an oil well with so much pressure that the oil shoots into the air without needing to be pumped.
"Verbal graffiti" is an interesting combination. It's not an idiom or a colocation. "Graffiti" is word--borrowed from the Italian--meaning words or pictures, written illegally on walls in public place. They are very often rude or obscene. Because graffiti are written, "verbal graffiti" is curious. A less creative way to say it would be "obscene words" or "bad language" or "curses." The writer wants to us to form an impression of what is said, based on the way graffiti look.
When he plays the tape, he releases a flood, a torrent, a gusher, a stream, a jet, a tsunami, of bad language.