If using a VBR compressed audio data format, you may want to instead use the AudioQueueAllocateBufferWithPacketDescriptions function. This function allocates a buffer with additional space for packet descriptions. The mPacketDescriptionCapacity, mPacketDescriptions, and mPacketDescriptionCount fields may only be used with buffers allocated with AudioQueueAllocateBufferWithPacketDescriptions.
The number of bytes of valid audio data in the audio queue buffer’s mAudioData field, initially set to 0. Your callback must set this value for a playback audio queue; for recording, the recording audio queue sets the value.