A headerless file of Dialogic/OKI ADPCM audio data commonly comes with the extension .vox. This ADPCM data has 12-bit precision packed into only 4-bits.
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.