The Speex is a patent-free, Open Source/Free Software voice codec.
Unlike other codecs like MP3 and Ogg Vorbis, Speex is designed to
compress voice at bitrates in the 2-45 kbps range.  Possible
applications include VoIP, internet audio streaming, archiving of
speech data (e.g. voice mail), and audio books. In some sense, it is
meant to be complementary to the Ogg Vorbis codec.

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