AMR codec source code and Standards docs.

Adaptive Multi-Rate (AMR) is a patented 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 and UMTS mobile phone standards.
AMR is also a file format for storing spoken audio using the AMR codec. Many modern mobile telephone handsets will allow you to store short recordings in the AMR format, both open source (see the external links) and commercial programs exist to convert between this and other formats such as MP3, although it should be remembered that AMR is a speech format and is unlikely to give ideal results for other audio. The common filename extension is .amr.
3GPP - 3rd Generation Partnership Project
Technical Specification Group Services and System Aspects
Performance characterization of the Adaptive Multi-Rate Wideband (AMR-WB) speech codec

Location of Standards Docs and Codec source code:

• http://www.3gpp.org/ftp/Specs/latest/Rel-5/26_series
• http://www.3gpp.org/ftp/Specs/archive/26_series/26.204/