I would like to also emphasize that the reason you cannot do this is a business decision that Apple made, not a technical decision. To wit, there was no rational technical justification for Apple to disable sound from web apps on the iPod Touch. That is a WiFi device only, not a phone that may incur expensive bandwidth charges, so that argument has zero merit for that device. They may say they are helping with WiFi network management, but any issues with bandwidth on my WiFi network is my concern, not Apple’s.
Also, if what they really cared about was preventing unwanted, excessive bandwidth consumption, they would provide a setting to allow users to opt-in to web apps sounds. Also, they would do something to restrict web sites from consuming equivalent bandwidth by other means. But there are no such restrictions. A web site can download huge files in the background over and over and Apple could care less about that. And finally, I believe the sounds can be downloaded anyway, so NO BANDWIDTH IS ACTUALLY SAVED! Apple just does not allow them to play automatically without user interaction, making them unusable for games, which of course is their real intent.
Apple blocked sound because they started to notice that HTML5 apps can be just as capable as native apps, if not more so. If you want sound in Web Apps you need to lobby Apple to stop being anti-competitive like Microsoft. There is no technical problem that can be fixed here.