I don't think you can change the pitch of an already playing sound without hacking some code and accesing the pool of sound sources directly.
But for now, you could try this:
[[OALSimpleAudio sharedInstance] playEffect:sfxFileName volume:1.0f pitch:globalPitch pan:0.0f loop:NO];
In your audio manager, you could store the current sound pitch in a float variable called _globalPitch. All sounds will play with the current value of _globalPitch (1.0f is the default). You could change the _globalPitch progressively in time (1.0, 0.9, 0.8 ...) with an scheduled function.
Is not exactly what you're looking for, but may work.