is the most common format for storing audio. Almost any player on any platform can open mp3 files. The audio is compressed with loss of quality, but the loss is negligible for the typical user, and the file size is usually less than that of the original files.
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.