同步声音与NStimer iphone sdk?

I am creating an application,in my app i am changing images for countdown.I want to play a Tick sound when one second completes(i mean when image changes).I have a 25 second long sound with repeated tick sound and within time interval of 1 second. sometimes it works fine but sometimes it's not.Now how do i sync audio with my timer.any help appreciated thanks.I am creating an application,in my app i am cha