Create a second wall. The construction of a typical interior wall transmits sound easily through vibration transference. Deaden this transference by building a second layer of wall around the room, ceiling and floor.
Measure 2 inches out from the existing walls. Build new wall frames made of 2- by 4-inch studs.
Install 5/8-inch drywall on one side of the frames before installing the walls permanently to the floor and ceiling. Extend any electrical outlets or switch plates.
Use wood screws to attach the frames to the ceiling and the floor, lining the top and bottom of the frames with a strip of 3/4-inch Styrofoam insulation glued to the 2- by 4-inch rail. These strips will help stop vibration from traveling through the frame to the ceiling and floor.
Fill the cavities of the new wall frames with loosely packed cotton batting or insulation. Do not use rigid foam products; you want more "air" in the insulation.
Cover the walls with 5/8-inch drywall. Glue the drywall to the frames with construction adhesive.
Paint the walls with Acousti-coat brand latex paint. This paint contains fillers that deaden transmission of sound.
Treat the ceiling and floor as if they were vertical walls and build a second layer of "wall" here too. Deck the floor with 3/4-inch plywood; glue it to the studs. Install industrial foam-backed carpeting.
Build "plugs" to cover any window openings. Create 1-inch by 3-inch frames to fit the windows. The fit should be snug so the plugs will stay in the windows without fasteners. Cover one side of the frames with 5/8-inch drywall. Fill the cavities with insulation. Cover the other sides with drywall. Install handles to make removal of the plugs easy.
Insulate under and around doorways to stop sound from leaking out of the rehearsal space. Install a hefty door sweep to self-seal the door when closed. Add strips of rubber or dense foam insulation around the insides of the door frame.
Retrofit secondary soundproof walls as portable units. Instead of installing these walls permanently to the ceiling and floors, wedge them into place. Build the secondary wall panels 1/8 inch to 1/4 inch too tall. Use wooden shims if necessary. Install handles to make removal easier. Remove the walls by popping them out of place.