hi! i am sure there are ways to implement volume sliders, but i didn't for this game and don't really plan on doing so retroactively. at least not anytime soon. the music isn't copyrighted or anything but i know youtube's systems are kind of silly regarding those things. you can really only mute the music entirely or turn the game's volume really low. sorry about this!
Answering your other questions below the cut:
the 'essence of community' is not a physical item-- it will not appear in your inventory. They're just progress flags related to one of the game's endings. So, yes, you can complete Chapter 2's secret quest without having completed Chapter 1's, you just have to figure out how. The guide has hints on the bottom if you want them.