The maud file extension is associated with the Amiga MAUD audio format, developed by Electronic Arts and Commodore in 1985. The MAUD is used for an 8 or 16 bit Amiga sound format. Most likely obsolete.
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.