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 is open lossless codecs, developed under the Ogg project. Lately it’s become quite popular: many users wishing to store audio with original quality choose FLAC. This is partly in connection with the fact that codecs uses IDv1 and IDv3 tags - this allows a user to sort an audio collection according to one’s particular tastes, changing the original metadata of the tracks.