How to mix audio played with FMOD.System.playSound()?

It looks like it’s version 2.02.20

I’m attempting to change the gain by using the slider in FMOD Studio’s Mixer panel while it’s connected to the Unity game.

Full disclosure, I’m not sure if what I’m trying to do should be working or not. I’m open to any other method to play a downloaded sound, but have it participate in the mix.