Any good C# library for signal processing?
I can recommend these libraries:

BASS - also has lot's of functions. Although it is not written for C#, they provide you with a wrapper (Bass.Net). Keep in mind that if you want to use this library in commercial project then you have to pay for the license.

NAudio - is a mature project with lots of functions and support for variety of codecs. 

cscore - rather new project with some nice capabilities, especially for visualizations, and some codecs that NAudio is missing. You might want to give it a try at some point, or even contribute to it.