Rick could leap off the stage and tackle this guy, but the guy is already retreating, and Rick is pretty sure beating the crap out of him would be boring. Or, alternately, the guy's tougher than he looks--he did come all the way over here just to tell them off, despite knowing Rick's reputation.

Anyway, there's a better method. The counterspell for a Silencing Charm is simple, but too hard to manage right now, so the alternative is just to completely override it. By using an Amplify Spell--which they've already been using, since actual amps won't work in Hogwarts.

So Rick taps his guitar a few times with his wand, mutters Sonorus three times, and then strums.

It's kind of like that scene in the beginning of Back to the Future where Marty turns the amps up basically way, way past eleven and just annihilates himself. A sonic boom issues from Rick's guitar, kind of like a lightning bolt just struck inside the room, and the shockwave knocks Rick over and literally everyone else in the vicinity. Anything made of glass shatters. The speakers blow out.

Rick lies in a heap and laughs his ass off, even though he kind of can only hear muffled sounds.

"T-Try that on for size, motherfucker!"