I've been a handyman for 12 years, and now a general contractor for 8 months. Ideally, first in the walls I'd put in R-19 w/sound dampening, next instead of drywall-skin the walls with at least 1/2" plywood-5/8" is better. Be sure to do this on the ceiling/roof as well! If that's still not good enough, really inexpensive way to go is buying a few cases of acoustic ceiling tiles (for grid/drop ceiling) liquid nails or just screw them covering the entire fascia of all your walls plus ceiling/roof. I guarantee nobody's going to hear nothing more than the low thrum....I'm doing this exact same identical thing in my backyard garage as we speak !!
