AMR files: It is basically a compressed audio format developed by Ericsson. Developed for GSM telephony & used by many 3G cell phones for voice recordings such as MMS messages. An audio file with the AMR file extension basically is an “Adaptive Multi-Rate” ACELP Codec file. ACELP stands for Algebraic Code Excited Linear Prediction (ACELP) algorithm, which is designed to efficiently compress human speech audio. NOTE: AMR files may be saved in either the AMR-NB (Narrowband) or AMR-WB (Wideband) format. Wideband audio supports a broader frequency range and therefore provides better sound quality.