We will add a Slider GameObject as a child to AudioOptions by right-clicking the AudioOptions GameObject and choosing UI | Slider from the drop-down menu.
We will then set 0 for min value and 1 for the max value, and leave deselected the Whole Numbers option to use float number in the range of 0.0 to 1.0. The Value slider can be set there and will change while our UI drives the change.
For this slider, we will make it super simple: we will specify the Soundtrack GameObject in the On Value Changed (Single) method and choose from the available object list: AudioSource and volume for its property.
Every time the slider will change its value by user input, the background music will adapt its volume accordingly:
The Slider component in the Inspector with the Soundtrack audio clip GameObject specified in the On Value Change event slot and the AudioSource.volume call set