This are 2 Bugs with the same slider
1. When you play a Disc in a noteblock and its on 100% and you slide it slowly down the sound does not go down only when it pas something like 25% then it just goes lower

2. if you slide the slider fully down till 0% and back up the sounds dont came back till you Replae the Disc in the Jukebox.
Here is also a video how it looks like. https://www.youtube.com/watch?v=b-d4_M9QEz8&feature=youtu.be