2 Mac Apps to Seperately Control Volume of Applications
Apogee Maestro Software
Volume Mixer Alternatives for Mac: Top 4 Volume Control Tools & Similar Apps | AlternativeTo
SoundDesk: Turn Your Mac Into A Virtual Mixing Desk | Radio.co
Pyle 5 Channel Audio Mixer - DJ Sound Controller Interface with USB Soundcard for PC Recording, XLR 3.5mm Microphone Jack, 18V Power, RCA Input and Output for Professional and Beginners -PAD20MXU :