These are some additional plugins for SoX.

Limiter is a hard limiter, it only works on 2 channel
audio files. It divides the waveform in chunks by 
searching for zero crossing points and applies the
limiting to these chunks.

Square fade is a patch for SoX fade engine that adds
a square fade, which I like very much to fade the end of songs.

SoX sources are needed to compile these additional plugins.
You should add limiter.c to SoX src directory and apply
the 3 patches provided.

These plugins are very experimental (expecially limiter),
use at your own risk.