Snd is a sound editor modelled loosely after Emacs. It can be customized and extended using either s7 (included in the Snd sources), Ruby, or Forth. Snd is free; the code is available via anonymous FTP as Snd tarball.

ccrma.stanford.edu/software/snd/
ccrma.stanford.edu/software/snd/snd/snd
sourceforge.net/projects/snd