You could have downloaded VolumeMixer 1.1.18 for Mac from our software library for free. This Mac app was originally created by Golden Goose Technologies. The software lies within System Tools, more precisely General.
From the developer:
Volume Mixer is an application that has the capability to change and adjust the volume control and its options on Mac OS Yosemite and El Capitan. Use the app preferences to change output devices, change the app look and feel and much more. Volume Mixer supports HDMI devices, among others.
You may want to check out more Mac applications, such as Display Rotation Menu, AppLaunch or ProcessWizard, which might be similar to VolumeMixer.