How to soundproof rehearsal room in shared building with other rehearsal rooms?
Since your limited to the type of construction you can perform on the walls; it would be best to consider a portable (or easily knocked-down) sound-lessening partition wall. My only thought is an 4' x 8' x 2 inch foam insulation board hinged to a second piece(for easy set-up). Setting up several of these against the existing walls should lower sound transmission in either direction.