I'm looking to soundproof my garage for recording purposes. Most recommendations suggest a 'room within a room' which should not be in any way connected with the outer garage wall. Insulated breeze-block could be OK and some suggest a stud wall using high-insulation boarding. Suggest the inner room is built four-sided with its own door to access garage doors which will also give greater security. For roof could use high density board plus acoustic tiles or foam blocks. Remember though that the noisiest instruments are bass and drums. So turn bass low and use damping on the drums or get an electo kit (Roland or Yamaha)