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.
This format was originally created as an alternative to mp3. The idea was to achieve a small file size with better sound quality. The first versions weren’t quite successful, but with the development of AAC, it became possible to store sound with less loss of quality, and with the same file sizes as that of mp3.