is the most common format for storing audio. Almost any player on any platform can open mp3 files. The audio is compressed with loss of quality, but the loss is negligible for the typical user, and the file size is usually less than that of the original files.
The AVR file extension is a data format known as Audio Visual Research File. AVR files were developed for use on Apple Macintosh computers. They were specifically, used for the older 680x0-based Atari ST computer systems.