A musty, dank smell coming from your walls is typical of mildew odour. Where there's mildew smell, there's usually mildew spores as well. Left unattended, walls that simply smell of mildew can develop mildew spores on the exterior of the wall as well as the interior. Rampant mildew growth can be dangerous to you and your family but is easily avoidable with cleaning and regular maintenance.
Pour 1 cup household bleach into a bucket with 1 gallon warm water. Stir the solution to mix.
Wear rubber gloves to protect the skin on your hands and arms from getting dried out from the bleach.
Soak a clean cloth in the bleach solution, then squeeze out the excess liquid. Wash down your walls where mildew odour exists. The diluted bleach will kill any existing mildew spores while removing the accompanying odour.
Allow your walls to air-dry. Open windows, when possible, to circulate fresh air in order to aid the drying process and remove strong bleach smell.
Remove lingering mildew odour on your walls with white vinegar poured onto a clean cloth. Wipe the smelly surface with the vinegar-soaked cloth, then let it air-dry.
Fresh air from open windows helps deodorise your walls and the rest of the room. Run a dehumidifier in damp, musty rooms to remove excess humidity. This helps get rid of mildew odours. Place a bowl of activated charcoal in the room with mildew-smelling walls. Leave it there overnight to absorb the odours.
If mildew odour persists after all of your cleaning and odour-removing efforts, call a specialist to make sure mildew hasn't got into the insides of the walls.