First, you must have the ability to turn Mario really small by hitting him with the hammer and you must have been through at least two green pipes. Go to a green pipe and have Luigi hit Mario to make him small. Then have Mario in the front while he is small and go through the pipe and click on another pipe to warp there. When you get out, you will hear the music that you heard from the other area, which you usually wouldn't hear in this area. For example, you can hear the Bean Bean Village music in Stardust Fields.
Submitted by: Gary DeWitt