AudioQueues can be used to record audio from the system input devices into a stream, and to play back audio. They are responsible for encoding your audio when recording or decoding your compressed audio when playing back and interact directly with the audio hardware and decoding hardware where appropriate.

AudioQueue is a base class for both the MonoTouch.AudioToolBox.InputAudioQueue which is used to record audio and the MonoTouch.AudioToolBox.OutputAudioQueue which is used to playback audio. This class provides services to start, prime, stop, pause the queues as well as volume control, resource management and event notifications.

When you use AudioQueues, you must allocate buffers for playback or recording. You use the MonoTouch.AudioToolBox.AudioQueue.AllocateBuffer method or the MonoTouch.AudioToolBox.AudioQueue.AllocateBufferWithPacketDescriptors to allocate them and you use the MonoTouch.AudioToolBox.AudioQueue.FreeBuffer to release them. You keep a collection of buffers around that the underlying hardware can use to either playback audio, or record into. As the buffers are used, a notification callback is invoked. In the OutputAudioQueue case, you connect to the OutputCompleted event to be notified when a buffer has been fully played back, and in the InputAudioQueue you use the InputCompleted event to be notified when a recording has fully utilized a buffer.

Unless otherwise specified, the callbacks for processing a filled audio buffer, or filling out an audio buffer are invoked on an AudioQueue thread. You can change this by providing an instance of the CFRunLoop that you want to use for processing the events in your queue.

When processing an input or output queue, you might want to listen to a few property changes raised by the queues during their processing (See the MonoTouch.AudioToolBox.AudioQueueProperty for a list of events that you can listen to). To do this, use the MonoTouch.AudioToolBox.AudioQueue.AddListener(MonoTouch.AudioToolBox.AudioQueueProperty, AudioQueuePropertyChanged) method to add a listener and use the MonoTouch.AudioToolBox.AudioQueue.RemoveListener(MonoTouch.AudioToolBox.AudioQueueProperty, AudioQueuePropertyChanged) method to remove the listener.

You can see the StreamingAudio to see how to use AudioBuffers.

The various AudioQueue properties are exposed as high-level C# properties. In addition to the high-level properties, a low-level interface to the AudioQueue property system is exposed in case Apple introduces a new property that was not previously bound or if you need finer grained control. The low-level interface is provided by the GetProperty and SetProperty family of methods.