Added duration property to BaseSound class

Author: pavle-goloskokovic

v3/src/sound/BaseSound.js

@@ -82,6 +82,14 @@ var BaseSound = new Class({
          */
         this.pan = 0;
         this.config = Extend(this.config, config);
+        /**
+         * A value representing the duration, in seconds.
+         * It could be total sound duration or a marker duration.
+         *
+         * @readonly
+         * @property {number} duration
+         */
+        this.duration = 0;
         /**
          * Flag indicating if sound is currently playing.
          *