Play video's audio track when enter background?

Question

I'm using mpmovieplayer to play video on ios 4, and I want to keep the audio playing when my app enter background, but it doesn't work. I thing it's because video player use GPU to render video on screen and this is not allowed by apple when app enter background.

So, is there any way to do that? There are some apps have this feature, and they seems just use the mpmovieplayer, is it work by detach the video layer of mpmovieplayer?

Thanks for your answer!