There is no wat to effectively 'soundproof' rooms in such a situation.
Effective soundproofing requires sustantial construction, way beyond anything you are likely to be able to achieve in a loft or bedroom. Best you can do is to try and work out how the sound is transfering from the bedroom to the hall way, for example, floor mounted speakers vibrating a suspended floor that is, in effect the ceiling for the space below might be the issue.
Try and minimise this transmission by moving or changing your speakers then damp the room as much as possible. Use carpets or rugs and other soft furnishings, if the problem is in the bass region, look at commercially available bass traps. In essence try and stop the energy getting into the structure of the building.
Good luck.