A very, very, very powerful multitrack audio processing tool for UNIX, and the library it uses (libecasound) that is also used in other tools. Written by Kai Vehmanen.
The program allows simple things, like playing the sound, and many very sophisticated sound changing things that involve chaining different effects.
This is the tool to be used when sox runs out of steam - though, if sox was tricky to use, Ecasound's syntax is probably even more confusing, though not impossible.
For example, here's how to do a fadeout around 220 seconds after the start of the sample. Don't ask how to interpret this, this was ages ago - I do remember that -kl2 did a linear fade and -f told the format of the output file.
ecasound -i input.wav -y:0.40 -ea:220 -kl2:1,100,0,185,5 -f:s16_le,1,44100 -o:output.wav
Home page: http://www.eca.cx/