Lprez: sadly, you can't play two sounds simultaneously, even with that method.
— : I tried to play wave file over an already playing audio background (tried with an .mp3 and a .mp4 on the ipad), and it doesn't work (I remember one of your comment saying that perhaps you would try wav SFX on top of compressed audio background). From my experience, it doesn't work, but I can be wrong in implementing that hack.
I also tried some javascript libs to generate wave audio data (like riffwave.js), it doesn't work either. And Mobile Safari doesn't want to play a wave or an mp3 base64-coded dinamically inserted into the DOM.
So I think we need to wait for Apple to change that on their browser, or find a XCode Project that uses a UIWebView and some tricks to bind the JS audio calls to some native code...