I found that there will be no sound in iOS 3.1.3 after I call iPhoneUtils.PlayMovie to play a movie(the movie still have sound).
Test on ipod 3Gen 8GB with iOS 3.1.3, but test on ipod 3Gen 32GB with iOS 4 have no problem.
My code like this:
if (nextLevelNo == 201 && GameState.inStoryMode == true)
{
yield return null;
iPhoneUtils.PlayMovie("part1.mp4", Color.black,
iPhoneMovieControlMode.CancelOnTouch, iPhoneMovieScalingMode.AspectFit);
GameState.gameStartLevelNo = nextLevelNo;
yield return null;
// fix bug: screen will darken after played movie
iPhoneSettings.screenCanDarken = true;
iPhoneSettings.screenCanDarken = false;
// display loading image
guiTexture.enabled = true;
Application.LoadLevel("GameScene");
}