Merge pull request #577 from matvelloso/master

Fixed small issue when calling dispose on Google Chrome

Babylon/Audio/babylon.soundtrack.ts

@@ -37,7 +37,9 @@
                 while (this.soundCollection.length) {
                     this.soundCollection[0].dispose();
                 }
-                this._outputAudioNode.disconnect();
+                if (this._outputAudioNode) {
+                    this._outputAudioNode.disconnect();
+                }
                 this._outputAudioNode = null;
             }
         }