Currently, the only workaround there is
to control Sound Volume via trackpad:
2 finger rotational action, plus jumps and tweaks regarding repeat steps (etc).
Expected: simple 3/4-finger scroll?
Being a software solution, can't BTT emulate any output needed, while reading simple n-fingers scroll action as imput?
So frequently used feature, that even win10 had it by default 
Same applies to Display brightness, maybe something else?