@riban is developing an Audio Mixer. It provides a fader for each engine you load to adjust the volumn as well as L/R balance, mute and solo buttons. It’s not in the master repository yet. For more info check here on the forum. New mixer proposal
There are currently some pitfalls when trying to get this to work. I suggest you read the entire thread before deciding if you want to take it for a test drive or give it some more time to be developed.