this is a proprietary format developed by Microsoft. Users have noticed low error resilience: sometimes insignificant damage to the file can cause the file not to play. Besides this, the closed format hampers its proliferation outside of Windows platforms.
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.