Sound.fadeOut(duration) will fade the Sound to a volume of zero over the duration given. At the end of the fade the Sound will be stopped and Sound.onFadeComplete dispatched.

Sound.fadeIn(duration, loop) will start the Sound playing, or restart it if already playing, set its volume to zero and then increase the volume over the duration given until it reaches 1. At the end of the fade the Sound.onFadeComplete event is dispatched.

Author: photonstorm

README.md

@@ -83,6 +83,8 @@ Version 2.1.2 - "Whitebridge" - in development
 * Rectangle.aabb is a new method that will take an array of Points and return a Rectangle that matches the AABB (bounding area) of the Points (thanks @codevinsky #1199)
 * AudioSprite support is now built into the Loader and SoundManager. AudioSprites are like sprite sheets, only they consist of a selection of audio files and markers in a json configuration. You can find more details at https://github.com/tonistiigi/audiosprite (thanks @codevinsky #1205)
 * Point.parse will return a new Point object based on the x and y properties of the object given to Point.parse (thanks @codevinsky #1198)
+* Sound.fadeOut(duration) will fade the Sound to a volume of zero over the duration given. At the end of the fade the Sound will be stopped and Sound.onFadeComplete dispatched.
+* Sound.fadeIn(duration, loop) will start the Sound playing, or restart it if already playing, set its volume to zero and then increase the volume over the duration given until it reaches 1. At the end of the fade the Sound.onFadeComplete event is dispatched.
 
 
 ### Updates

src/sound/Sound.js

@@ -242,6 +242,11 @@ Phaser.Sound = function (game, key, volume, loop, connect) {
     */
     this.onMarkerComplete = new Phaser.Signal();
 
+    /**
+    * @property {Phaser.Signal} onFadeComplete - The onFadeComplete event is dispatched when this sound finishes fading either in or out.
+    */
+    this.onFadeComplete = new Phaser.Signal();
+
     /**
     * @property {number} _volume - The global audio volume. A value between 0 (silence) and 1 (full volume).
     * @private
@@ -775,6 +780,74 @@ Phaser.Sound.prototype = {
 
     },
 
+    /**
+     * Starts this sound playing (or restarts it if already doing so) and sets the volume to zero.
+     * Then increases the volume from 0 to 1 over the duration specified.
+     * At the end of the fade Sound.onFadeComplete is dispatched with this Sound object as the first parameter, 
+     * and the final volume (1) as the second parameter.
+     *
+     * @method Phaser.Sound#fadeIn
+     * @param {number} [duration=1000] - The time in milliseconds during which the Sound should fade in.
+     * @param {boolean} [loop=false] - Should the Sound be set to loop? Note that this doesn't cause the fade to repeat.
+     */
+    fadeIn: function (duration, loop) {
+
+        if (typeof duration === 'undefined') { duration = 1000; }
+        if (typeof loop === 'undefined') { loop = false; }
+
+        if (this.paused)
+        {
+            return;
+        }
+
+        this.play('', 0, 0, loop);
+
+        var tween = this.game.add.tween(this).to( { volume: 1 }, duration, Phaser.Easing.Linear.None, true);
+
+        tween.onComplete.add(this.fadeComplete, this);
+
+    },
+
+    /**
+     * Decreases the volume of this Sound from its current value to 0 over the duration specified.
+     * At the end of the fade Sound.onFadeComplete is dispatched with this Sound object as the first parameter, 
+     * and the final volume (0) as the second parameter.
+     *
+     * @method Phaser.Sound#fadeOut
+     * @param {number} [duration=1000] - The time in milliseconds during which the Sound should fade out.
+     */
+    fadeOut: function (duration) {
+
+        if (typeof duration === 'undefined') { duration = 1000; }
+
+        if (!this.isPlaying || this.paused || this.volume <= 0)
+        {
+            return;
+        }
+
+        var tween = this.game.add.tween(this).to( { volume: 0 }, duration, Phaser.Easing.Linear.None, true);
+
+        tween.onComplete.add(this.fadeComplete, this);
+
+    },
+
+    /**
+     * Internal handler for Sound.fadeIn and Sound.fadeOut.
+     *
+     * @method Phaser.Sound#fadeComplete
+     * @private
+     */
+    fadeComplete: function () {
+
+        this.onFadeComplete.dispatch(this, this.volume);
+
+        if (this.volume === 0)
+        {
+            this.stop();
+        }
+
+    },
+
     /**
     * Destroys this sound and all associated events and removes it from the SoundManager.
     *