It could be carrying along the adjoining walls, through the subfloor via deliberate vent gaps or accidental gaps, or through the ceiling/floor void above. Possibly also via the chimney.
Just don't waste your time. If it bothers you and there's no big reason to live there then don't. You'll throw vast amounts of money and time at it, make your room smaller and probably achieve little or nothing. Then your buyer might want to know why you needed to soundproof the place, it could make them think they're buying a house with problem neighbours who scream and fight all night.
Whoever you sell to probably won't care, many people don't. Or they'll make so much noise your next-door neighbours will move.