So I'm using slider bars to control the volume but the volume isn't changing, how do I fix this?
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You are setting the volume before you play the music. When you start the music it is resetting the volume (to 1 dB).
rather than do it in two actions you could do it in one:
Audio - play MainTheme looping at volume MusicBar.Value dB (tag 'music')
construct.net/en/make-games/manuals/construct-3/plugin-reference/audio
Sound doesn't go up, it goes down! Highest volume is 0db. To adjust you have to go negative.