The way you currently have implemented this will cause our Music to Play each time the slider value changes. What you need to do instead is to call PlaySound at Location on event begin play, and then just set the variable VolumeMultiplier by calling the setter from the Music variable.
Also, I am assuming you want to play a background music for your game, therfor I would recommend using PlaySound2D instead, as play sound at location will spawn the sound at a location (in this case 0,0,0)
If you want to be able to control the volume of the music you could set up a sound cue with a continuous modulator, you could then control the volume of the music outside of the sound cue once spawned.