Apps with 'Virtual Mixer' feature

Voicemeeter is Audio Mixer Application endowed with Virtual Audio Device used as Virtual I/O to mix and manage any audio sources from or to any audio devices or applications.

SoundDesk is the easiest way to turn your macOS into a professional mixing desk.

Displays information and current volume level for all active sound components on your system, enables saving a sound profile into a shortcut file to load later, or changing individual settings via a shortcut file with extensive command line support.