You can set the max and min value of the slide bar to whatever value you want, so set the min value to -20 or -30 I don't know exactly, and the maximum to the same thing.
Not that 0 is the original volume of the sound or song you're playing so set 0 to the maximum of the slide bar depending on what you want to do.
If you play a song on the start of every layout, a song will be played every time you change layouts, but this doesn't mean the song that was playing previously will stop.
To avoid to songs playing at once, on the start of the layout, on the same condition that you start playing the song, create a event to stop playing the song that was playing previously.
Just because the song is mute doesn't mean it's not playing.