The single most cost-effective thing I did was replace the garbage hollow-core interior doors with nice hefty solid-core doors. Hollow-core doors are common for

The single most cost-effective thing I did was replace the garbage hollow-core interior doors with nice hefty solid-core doors. Hollow-core doors are common for interiors since they're cheap -- but they also act effectively as sound amplifiers since they're just two vibrating planks with a small air gap. Solid-core doors -- even ones filled with dense fiberboard (vs pure solid wood) are VERY effective at reducing sound. And they're cheap -- you can get a nice basic one for ~$100. If you pad the door frame with $5 in closed-cell weatherstripping and put in a $10 door sweep at the bottom (creating a nice seal around the door), you've eliminated one of the lowest-hanging fruits of sound transmission for very little $. Next on the list: Green Glue. If you don't mind a bit of messy DIY stuff, you can make your own drywall/Green Glue sandwich for a very reasonable amount of $. Doors & Walls cover about 80% of sound issues, and it doesn't have to be expensive.