added duration description

Author: pavle-goloskokovic

v3/src/sound/BaseSound.js

@@ -25,6 +25,12 @@ var BaseSound = new Class({
          * @property {ISoundConfig} config
          */
         this.config = {
+            /**
+             * A value representing the duration, in seconds.
+             * It could be total sound duration or a marker duration.
+             *
+             * @type {number}
+             */
             duration: 0
         };
         /**