I have a Behringer USB audio interface with multiple outputs (headphones and speakers), but I only have one master volume control in the audio settings and in the system tray. The sound is currently output to both, the speakers and headphones simultaneously. With other distros I was always able to choose and adjust the output in the tray.
The outputs are available in pavucontrol and can be set for left and right. That means they work, but they are simply not visible in the system.
How do I get the outputs for headphones and speakers into the tray so that I can easily change and adjust them from there?
The other distros properly have newer version of Gnome than you find find in Zorin OS which goes for stability over bleeding edge - however you can try this, a gnome extension that allow you to adjust/select and hide audio(s): Quick Settings Audio Panel - GNOME Shell Extensions
You could also try to install all ALSA packages using Synaptic Package Manager. Also in Audacious, use the SoundCard settings as how I have laid out here: