Libnyquist is a library interface to Roger Dannenberg's LISP based audio processing language. It's home page is here. You can write scripts to process your audio, and feed them directly from the audacity interface. This is used to implement a number of effects plug-ins shipped with audacity, as well as the extensive list of plug-ins on the main audacity website. An introduction to creating nyquist plug-ins can be found on the main audacity website.