I tested a simple app to start playing sound effect immediately at the start. Tested on Windows phone 7, and PC seems to be fine.
But on iOS (on iPad2), no sound effect is played.
I checked it by testing against a testcase (http://www.cocos2d-x.org/cocos2d-html5/index.html), and it plays sound fine.
You can test my simple app at http://tkappport.comli.com/prototype/. It will render background and play a sound effect.
Any suggestion on what might be a problem?