Greasy Fork

doesn't work, fix included

The four lines in the code:
$("#audioSource").volume

should be instead:
$("#audioSource")[0].volume

additionally, the element isn't ready until the document is loaded, so the initial volume setting will generate an error. So change the first volume setting:

//Sexy Script!
$("#audioSource").volume = percentage/100;

to:
//Sexy Script!
$( document ).ready(function() {
$("#audioSource")[0].volume = percentage/100;
});