I'm not sure I understand correctly. You want for example the sound fx and music play at different levels?
If so, then keep the master volume at db 0 and don't use a slider for that.
Instead, create 2 sliders that each one will control one of the 2 categories (fx or music). Whenever you trigger a new sound, pass the corresponding slider value into its "Volume" parameter.
Play "MyMusic" looping at volume Slidermusic.Value
Play "MyFX" not looping at volume Sliderfx.Value