After opening an audio unit (IAA) using an AUGraph or directly with AudioComponentInstanceNew on an iPhone with a fixed hardware sample rate of 48000Hz, AudioUnitInitialize() will fail when either or both of the input and output AudioStreamBasicDescription's set for the audio unit contain a different sample rate (like 44100Hz). Setting the formats themselves return no error.The question is why is this behavior? It doesn't seem to be IAA specific, since I tested many. One could also use the IAA in an offline setting, with a generic output AU for example, totally unrelated to the actual audio hardware.