Sound proofing a waste pipe
If the pipe is exposed, you can enclose it with a small wall (several 2x4's for the framing and a little drywall).  Make sure at any cleanout openings, you include an access panel in the wall.  Then, before you install the drywall, put a bunch of insulation in there, surrounding the pipe.  Avoid having anything solid connecting the pipe to the wall since that will transmit sound.

Edit: A post construction idea is to hang something that absorbs sound on the wall, such as an acoustical panel frequently used for home theaters or even a tapestry.