I worked on 2 houses and partially a 3rd where the buyer asked for a 'man cave' room. We'd put nicer insulation on all sides and slightly gap the board from the wall (hard to explain) then double layer the drywall. We'd use thicker board typically used on ceilings for all walls, and carpet the room (makes a difference), along with a solid door. A good window if it had a window, like double pane or a window marketed to be sound proof.
Then most importantly we'd go around the cracks of the room with a can of foam insulation and seal every hole we could find.
These rooms where pretty quiet. I can't say how much of a difference just the drywall would be, but if I wanted the cheapest way I'd do foam sealing and double drywall. Tearing everything out to replace behind wall insulation would only be something I'd do if there was zero insulation in the walls. I guess you could tell by knocking the walls with your hand. If they sound hollow you'll know. If you have a friend with a sound proofed room ask to walk around and get a feel for the space so you'll know what to aim for.
Drywall is easy to replace though and something more diy-ers should try more often imo!
Then most importantly we'd go around the cracks of the room with a can of foam insulation and seal every hole we could find.
These rooms where pretty quiet. I can't say how much of a difference just the drywall would be, but if I wanted the cheapest way I'd do foam sealing and double drywall. Tearing everything out to replace behind wall insulation would only be something I'd do if there was zero insulation in the walls. I guess you could tell by knocking the walls with your hand. If they sound hollow you'll know. If you have a friend with a sound proofed room ask to walk around and get a feel for the space so you'll know what to aim for.
Drywall is easy to replace though and something more diy-ers should try more often imo!