This lets the user drag the jPlayer volume bar instead of just clicking a new volume value. Substitute #player and .jp-volume-bar-value with your own selectors. If you are using a horizontal volume bar, you'll have to hack in negative values instead.

jquery.jplayer.draggablevolume.js

$('.jp-volume-bar').mousedown(function() {
    var parentOffset = $(this).offset(),
    width = $(this).width();
    $(window).mousemove(function(e) {
        var x = e.pageX - parentOffset.left,
        volume = x/width
        var barValue = Math.floor(volume*100);
        if (barValue < 0 ) barValue = 0;
        if (barValue > 100) barValue = 100;
        jQuery(".jp-volume-bar-value").css("width", barValue + "%");
        $("#player").jPlayer("volume", volume);
    });

    return false;
})
$(document).on("mouseup", function() {
    $(window).unbind("mousemove");
});