The Speex project aims to build an open-source patent-free voice
codec. Unlike other codecs like MP3 and Ogg Vorbis, Speex is specially
designed for compressing voice at low bit-rates for applications such as
voice over IP (VoIP). In some sense, it is meant to be complementary to
the Ogg Vorbis codec.

WWW: http://www.speex.org/