There are plans for a noise cancellation headphone in the library. Search this forum for tips from people who've built it. If you go ahead with the project, pls post details as you proceed in the DIY forum. A noise cancellation steth could have many applications, such as automotive diagnostics. I seem to recall that service garages use some kind of steth to diagnose engine malfunctions.
I do caution that when you take into account the cost of parts and time, $300 may not seem like too much money.
[Edited by cmoy on 02-11-2009 at 08:39 PM.]