Standard pink fiberglass insulation does not work for soundproofing; it's too fragile. Your best bet is probably to add a freestanding wall inside the garage. As for the crack under the garage door, if you're not going to use the garage for a car, you could use a combination of sandbags and towels. Cardboard is one cheap option that can be added to an amature project to assist the soundproofing. If you can find any carpet or carpet underlay, that works pretty good too.