I'm afraid NAudio doesn't come with a bandpass filter (I'd like to include some filters in the future). What I normally do in cases like this is find some C code that does the filtering (there are good sites out there that will work out filter coefficients and even generate the code), and then port it to C#. If you are using the ISampleProvider interface, this should be quite straightforward, as you'll just be processing samples as 32 bit IEEE floats as they come through your Read method.
Mark