My way of blocking out all outside noises whilst listening to paraliminals and other self help tapes is to wear ear plugs in my ears, then slip on my headphones, then turn up the volume a bit...and voila! Virtually all outside noises are blocked out and yet I can hear the paraliminal perfectly. It really is very effective, and is much less costly than buying noise cancelling headphones.
I buy my earplugs from shops selling motorbike equipment (they recommend you wear earplugs whilst on a bike because it can get very loud) and they work out very cheap.
Try it and see what you think.