Adaptive Multi-Rate (AMR or AMR-NB) audio codec is a proprietary audio compression technology optimized for speech coding. AMR codec has been adapted for use in speech applications by 3GPP consortium (3rd Generation Partnership Project) in October 1998 and now widely used in GSM and UMTS. Encoder adapts to the quality of communication and uses the most appropriate bitrate based on current conditions.
Many modern phones allow you to store short audio recording in the AMR format. There are both free and commercial software to convert, but it should be remembered that AMR does not provide a good quality for non-voice content.
With Audio Converter Plus you can: