Audioオブジェクトでフェードアウトする

環境

chrome 44.0.2403.130 m (64-bit)

example

一定間隔でフェードアウト

volume = 1 - a*frame

music = new Audio();
music.src = "your/music/path.mp3";

music.fadeTimerId = null;
music.fadeOut = function (time) {
    var frame = 30;
    var frameCount = 0;
    var endFrame = time * frame;
    var framePerSecond = 1/frame;

    var start = this.volume;
    var step  = this.volume / (endFrame);
    
    var that = this;
    that.fadeTimerId = setInterval(function(){
        var nextVolume = start - step*frameCount ;
        if (nextVolume <= 0) {
            clearInterval(that.fadeTimerId);
            that.pause();
            that.volume = 1;
            return;
        }
        that.volume = nextVolume;
        ++frameCount;
    },framePerSecond * 1000);
}

滑らかにフェードアウト

volume = 1 - a*frame²

music = new Audio();
music.src = "your/music/path.mp3";

music.fadeTimerId = null;
music.fadeOut = function (time) {
    var frame = 30;
    var frameCount = 0;
    var endFrame = time * frame;
    var framePerSecond = 1/frame;

    var start = this.volume;
    var step  = this.volume / (endFrame*endFrame);
    
    var that = this;
    that.fadeTimerId = setInterval(function(){
        var nextVolume = start - step*frameCount*frameCount ;
        if (nextVolume <= 0) {
            clearInterval(that.fadeTimerId);
            that.pause();
            that.volume = 1;
            return;
        }
        that.volume = nextVolume;
        ++frameCount;
    },framePerSecond * 1000);
}