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");
    }