> what's the difference between AHI music unit and individual AHI units?

See comment #54.

> I checked mplayer's preferences and couldn't find any option regarding
> how it's configurated (only option is Sound Driver: AHI).

"Two audio drivers are available : ahi and ahi_dev. "ahi" is default driver and uses music unit (exclusive mode). "ahi_dev" uses device mode (means it can be shared). If you use ahi device.audio, then exclusive mode is rerouted to device unit, so these audio settings won't really matter.
Syntax: -ao <driver name>
Example: -ao ahi_dev"
https://www.morphos-storage.net/Multimedia/MPlayer_1.3.0-20181121.readme