How to muffle your piano from your neighbors and still hear what you're playing:
"Without knowing what kind of piano you have (upright?) there is another solution - you hang a damper quilt (looks like a quilted moving blanket) on the back of the piano and let the bottom of this go under the piano. This prevents the piano sounds from bouncing off other surfaces and should reduce some of the sound. You can still hear what you are playing when you use this. Yamaha used to sell them with the upright studio line."
--posted by Alexalb
Read the entire thread here.