Speex

spx	This is an audio codec. The name to use at the command line is spx. This codec can be used inside the Ogg containers. This codec is from the speex module.

Speex is a deprecated speech-oriented audio codec designed for use over packet networks (e.g. HTTP, local network use) and VoIP applications. As of 2013, Speex.org recommends parties consider Opus instead, calling the Opus codec superior.

It supports compression in a lossy way. It uses CELP (Code Excited Linear Prediction) to compress, meaning it takes advantage of the frequency patterns of human speech to efficiently compress data; this also means harmonics that do not follow this pattern will perform poorly (e.g. electronic music). VLC media player has supported decoding and encoding Speex since version 0.7.0.