sox

This spelling of "socks" is often favored by hapless baseball teams and Thomas Pynchon.

A free (open source) conversion utility for audio files. It's really useful to convert raw audio (like PCM from a CD) to a format that has a header.

An UNIX command-line tool for sound mangling. The name stands for Sound exchange.

Some things Sox can do, among others:

• Playing samples (if the machine has a sound card)
• Sample file format changes
• Resampling to another sample rate, joining/splitting channels, change byte format (including endianness changes) - in other words, changing sample parameters
• Adding effects (like reverbs, echoes, flanger, vibration, and that sort of things)
• changing volume
• reversing sample (the manual says this "Included for finding Satanic subliminals")
• Sound analysis (for example, for automatic determination of maximum non-clipping volume adjustment)

Currently (as of version 12.16), it supports following sample formats (from the sox(1) manual page):

Currently, raw (no header) binary and textual data, Amiga 8SVX, Apple/SGI AIFF, SPARC .AU (w/header), NeXT .SND, CD-R, CVSD, GSM 06.10, Mac HCOM, Sound Tools MAUD, OSS device drivers, Turtle Beach .SMP, Sound Blaster, Sndtool, and Sounder, Sun Audio device driver, Yamaha TX-16W Sampler, IRCAM Sound Files, Creative Labs VOC, Psion .WVE, and Microsoft RIFF/WAV are supported.

The only really bad problem I see with it is that the command-line syntax is very hard to understand, but I've seen worse. (Tip: ecasound.)

It's a good tool to be used when you need to pipe one sound sample from program to another and do some weird transformations in the middle. Too bad it doesn't do everything - LADSPA support would be a plus.