iOS AVAudioPlayer audio fade in.

gistfile1.m

        soundPath = [[NSBundle mainBundle] pathForResource:@"soundtrack" ofType:@"m4a"];
        soundtrack = [[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL fileURLWithPath:soundPath] error:nil];
        [soundtrack setVolume: 0.0];
        [soundtrack prepareToPlay];
    }
    return self;
}

-(void)doVolumeFade {
    if (soundtrack.volume < 0.1) {
        soundtrack.volume = soundtrack.volume + 0.01;
        [self performSelector:@selector(doVolumeFade) withObject:nil afterDelay:0.1];
    }
}