Intelligent hearing aid
Ojoshi at instructables.com has posted a manual on how to build this arduino based hearing aid system.
From his instructables page:
it has tuning functionality that allows the wearer to tune the amplification to his or her needs. It has a conversational mode which recognizes voice input and amplifies it while reducing background noise. It saves all data to memory so that the device can be quickly powered up and ready to use. This device also has a very easy user interface to keep operation quick and simple.
If you are going to try and build this, take maximum care and do it at your own risk!