It seems that when they carved out this guest unit they failed to insulate the walls. I've read up on spray foam insulation, adding drywall over existing drywall with green glue in between.
Both sound like quite a project for a lay person. Although, I do have a friend in construction who has agreed to help with another layer of drywall. I am just not sure what will add maximum sound barrier.
Do I need to drywall the closet ceiling too since it's the closet through which I hear the most noise. (Help! It's like living in the same room with the tenants in the main house).
Best Answer
Sound proofing is difficult.
One extra layer of drywall and green glue will have a noticeable effect but don't expect it to be dramatic.
To give you an idea. I have a laminate with underpad, 2" concrete floor, 2x10 floor assembly, roxul batt insulation, 5/8" drywall between me and people living under me. This deadens noise such that I can't understand conversations - I can hear them they sound as if someone might be outside walking by having a conversation.
It would help if you drew a plan view of how your guest room is craved out. The best solution for this would be to build a extra wall off set from the existing one that separates the spaces, fill the wall with roxul insulation (you want mass), and drywall it. Depends if you can stand to loose 4.5" of your room.