A soundproof box can be made for musicians by finding a room with a thick door, soundproofing the edges of the door and placing soundproof MDF or sealant in any open areas around the room. A musician would also want to build a raised floor to help reduce the potential for sound to travel via the bass or speakers through the floor beams.
Making a room within a room is the one thing that will help keep a larger room soundproofed. The new room has new walls that can be made of MDF if a person is on a budget, or for high-quality walls that don't allow sound through, concrete blocks may be used on top of neoprene to prevent the blocks from touching the floor.
The new walls help reduce vibration and sound escape. It's important that the person building this room should make the box a standalone, meaning it should not be connected to the outside room's walls or ceiling. The floor should be raised and insulated with an acoustic insulator if possible. Mineral wool can be used as an acoustic insulator between MDF walls and in the ceiling of the room.
On the floor, wooden beams may be run along the original floor. These boards should have neoprene strips between them. A chipboard floor can be used to finish the floor by placing it on top of the beams.
The person building this room should prevent gaps by making only small holes for extension cords. The hole should only be large enough for the cord but not the extension plug because this would let sound through. The hole should be sealed afterwards.