In Foobar player: changing the volume in knob does not change the volume bar (ui) in Foobar.
(this is something that works in VLC player)
(Very low priority.)
This is related to how both softwares handle volume change. VLC is changing it's volume in the Windows mixer whereas Foobar has it's own dedicated internal volume system.
For MIDI Mixer to change the built in level slider of Foobar, a plugin would have to be made.