The M music format was created by Origin Software and used in at least three of their games. It stores audio data that is sent, via drivers, to various devices like AdLib, MT-32, Game Blaster, the Tandy speaker, and a couple others.
The format is compressed using an LZW algorithm.
You can decompress the files using U6Decompress by Glanestel Dragon (include source).
This format has been used in the following games:
Although the Worlds of Ultima games use the Ultima 6 engine, they do not use this audio format.