3GPP floating-point Adaptive Multi-Rate (AMR) Speech Codec
(3GPP TS 26.104 V 6.1.0)
http://www.3gpp.org/ftp/Specs/html-info/26104.htm

AMR is an audio data compression scheme optimized for speech coding.
AMR was adopted as the standard speech codec by 3GPP in October 1998
and is now widely used in GSM.

Note that depending on law in your country, you may need a license for
distribution, any use, commercial use or even development.

WWW: http://www.penguin.cz/~utx/amr