If you are experiencing sound transfer from one bedroom to another, or from the family room to a bedroom, it's time to consider soundproofing.
The soundproof characteristics of a wall are rated by sound transmission class (STC). Normal conversation will be heard and understood through an STC-25 wall. When soundproofing is improved to STC-60, loud shouting can be heard, but not understood.
Interior walls with a single layer of drywall nailed to two-by-four studs have an STC of about 35. Loud talking can be understood through the wall. In older homes, gaps and openings make things even worse.
The minimum recommended STC for a bedroom wall is 48, assuming that the adjacent room isn't noisy. For a bedroom wall adjacent to a kitchen or family room, a minimum STC of 52 is recommended and 58 is optimal.
With an existing wall, adding another layer of 1/2-inch drywall over the existing wall helps substantially with minimal floor space loss. It will also seal gaps and direct air paths where sound waves move easily. Hang the new drywall in place, possibly with some cushioning sheet (cork) under it.
If you can handle losing a little more floor space, nail a layer of resilient channels on the original wall first. The new layer of drywall will be attached to these soundproofing channels.
A better, though more expensive option is to attach special sound barrier fiberboard made from recycled newspapers.