| If isolation is an issue, building each room as a "box inside of a box" and de-coupling each room from the floor, walls and ceiling of the main building, as well as any mechanical systems is the right way. Framing the walls with 2x4s on a 2x6 or 2x8 plate allows using a dense membrane and fiberglass insulation keeps the inside of the wall isolated from the outer surface. This works very well. Also, using a heavy, windowless door with weatherstrip helps and don't use one door from one room to the other- if each room has a door, it will attenuate the sound even more. If a window is needed so people can see from one room to the other, make sure the frames aren't connected to both sides of the wall- keep those separate, too. Use insulated glass and angle them (at different angles) so any sound getting through will have minimal effect.
If you want to do it as cheaply as possible and doing the "box in a box" can't happen, peel the drywall on one side and insulate with fiberglass batts, then install new drywall. If you can, add a second layer of drywall on each side, using Green Glue to bond it to the studs and first layer. | 
