Hang heavy curtains that are made from a thick upholstery fabric over the windows. Another fabric option is thick velvet, which is often used in movie theaters. You can add these draperies along the length of one or more walls to further dampen sound.
Cover large areas of solid floors with carpeting or area rugs.
Add furniture to break up and absorb the sound waves. For bedrooms, upholster the headboard to further reduce echoes. Make the bed with both sheets and a comforter, and place pillows vertical against the headboard.
Add a bed canopy over a bed or attach a colorful tapestry to the ceiling with push pins or hooks. The fabric will absorb any sound bouncing around tall ceilings.