iphone: playing audio playlist in the background?

问题

I'm trying to play sequence of audio files in the background using AVAudioPlayer. When the app is not in the background it plays well, but when the app goes to the background it won't play the second song.

Here is my code:

-(void)playAudio:(NSString *)path{
    NSURL *url = [NSURL fileURLWithPath:path];

    NSError *error;
    AVAudioPlayer *ap = [[AVAudioPlayer alloc] initWithContentsOfURL:url error:&error];
    [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error:nil];
    [[AVAudioSession sharedInstance] setActive: YES error: nil];
    ap.delegate = self;

    mainController.audioPlayer = ap;
    [ap release];


    if (mainController.audioPlayer == nil)
        NSLog([error description]);
    else{


        UIBackgroundTaskIdentifier newTaskId = UIBackgroundTaskInvalid;

        [mainController.audioPlayer prepareToPlay];

        if([mainController.audioPlayer play]){

            newTaskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:NULL];

        }

        if (newTaskId != UIBackgroundTaskInvalid && bgTaskId != UIBackgroundTaskInvalid)
            [[UIApplication sharedApplication] endBackgroundTask: bgTaskId];

        bgTaskId = newTaskId;
    }

}

- (void)audioPlayerDidFinishPlaying:(AVAudioPlayer *)player successfully:(BOOL)flag{
    NSLog(@"finished now next...");
    [self playAudio:[self getNextAudio]];

}

I debugged the app and it seems that when it's about to play the second song [mainController.audioPlayer play] returns NO, which means it can't play. So what do you think?

UPDATE

After some testing it seems that it does continue to play properly only if the device locks, but if the user presses the home button and the app goes to the background the problem still remains