This mixer plugin may be used instead of the traditional global
software mixer. It integrates with the "volume" filter plugin, and
can control the software volume of an audio output which has no
hardware mixer.
The "volume" filter plugin will replace the current software volume
code. One "volume" filter may be attached to each output device.
This will allow the user to use hardware mixers for some devices, and
software mixers for other devices at the same time.
Currently, neither the filter API nor the "volume" plugin is
integrated into MPD.
