Building materials include a dampening layer which contains a plaster and a viscoelastic polymer, such as polyvinyl butyral. The dampening layer may also include a barium salt. Methods of making a sound dampening material include providing a plaster mixture. The mixture may include a viscoelastic polymer and/or a barium salt. The plaster mixture is combined with water and/or a viscoelastic polymer dispersion, to form a slurry. The slurry is applied to a surface and set to form a sound dampening layer.