Is there something I can put in the walls, it's going to be a 11.25ft x 13.75ft closed room.

Is there something I can put in the walls, it's going to be a 11.25ft x 13.75ft closed room.

Do you mean, "is there something to sound proof the room", so that people on the outside can't hear what's going on inside, and outside noise doesn't make it's way in?

Is there something I can put in the walls, it's going to be a 11.25ft x 13.75ft closed room.

Walls always benefit from being filled internally with fluffy fiberglass. This reduces resonance you hear inside the room, and also improves sound isolation between rooms. But it's not a substitute for acoustic treatment inside the room which you also need. Much more here:

Acoustics FAQ (http://www.ethanwiner.com/acoustics.html)

--Ethan

Agreed. Insulating walls helps them to not act like a big, ringing drum and also provides some benefit in bottom end control - though not sufficient nor broad enough to do the entire job.

Bryan