HI Billy! A great question and here are some information links for your project. Good luck to you.
One thing a family member does is to use one of those draft-stoppers (easily purchased at BB&B and other home stores), at the bottom of the door. It's amazing how much noise was transmitted through that tiny space at the threshold. The draft-stopper, placed up against the bottom of the door when the room's occupant goes to bed, helped muffle the noise tremendously.
You can cover the hard surface with something soft. I would start by taking the door down to work on it flat. Remove the hardware. Find a fabric that you like but make it one that is not too heavy. I would attach a layer of foam (about 1" thick) with spray adhesive. Cut it back so that all of the door knob will sit inside the foam when the door is re-hung. Next lay a piece of polyester batting over the foam and trim it so it doesn't hang over the sides of the door. The real work comes when you lay tbe fabric over the batting and staple it in placeplace. Put some staples around the cutout foam at the door knob location but don't cut the hole in the fabric yet. Put a staple in the center of the top about midway thru the doors thickness. Do one in the bottom and one in each side. You want your fabric to be tight but not stretched. Continue placing your staples opposite each other until the fabric is completly attached. You may need to cut and fold the fabric so as not to interfere with the hinges and a little glue can be helpful there as well as the corners. Once you have the door covered trim the fabric and cut out for the doorknob. Your door should be ready to re-hang.