Well, I can sort of do it. I have a canvas with buttons, text and sliders on it. When your press the button, it pops up with time scale 0 (not really a pause but I'll fix that). This is just activating the Ui canvas and setting time scale to 0. Then when pressing the button it deactivates the menu and puts time scale to 1.
I just don't know how to use the slider bar to set the music volume. I'm using master audio which has settings. It's using the slider part I don't know how to do.