Here's the deal, build a wall just on the inside of the garage door and attach some hinges to it so it then becomes a very thick, heavy gate. it can open in or out, just as long as it can be demonstrated to open far enough for a small car to enter the garage. Once it is up, you can place neoprene, insulation or any foam barrier around it to get it air tight.
Use Z-channel, 2 layers of sheetrock, and lots of caulk on all 3 walls and parts of the gate. And don't forget to keep sound from going out the roof. BTW, stopping any loud low frequencies from getting through the walls is very tough, basicly takes a lot of mass and is not worth addressing in your situation.
Also, you need dampened ventilation/hvac.
When done properly, you should be able to get 60db of attenuation down to about 500hz. Attenuation drops drasticly below that.
