Not respecting the ring/silent switch is the second most common audio related complaint I see from users in reviews and on forums like Touch Arcade (the most common complaint is not being able to play their own music).
The Apple Human Interface Guidelines explicitly state that when the silent switch is on the user should:
"Avoid hearing game sounds, including incidental sounds and soundtracks, that are not essential to using the game."
So if you don't do this you risk your app being rejected in review because it did not follow the HIG.
However, if you still want to proceed with ignoring the silent switch - it is the audio session category that determines the behaviour of the app with regard to the silent switch. You need to find an audio session category that ignores the silent switch and set your app to use that audio session category. From memory though I don't think any of the categories that ignore the silent switch will allow the user to listen to their own music so you'll risk annoying users on two fronts.