do the lock sounds on anyone else's iphone go away after a period of time? i'm talking about both the sleep/wake button (when u PUT it into sleep mode) and when u slide to unlock the phone. sometimes only 1 will work and sometimes neither work. just annoying, that's all. and yes, i've made sure that it wasn't on vibrate and the setting under "sounds" is enabled. i've actually turned it off then back on and SOMETIMES the sleep/wake sound comes back but never the unlock sound.