Feel free to post new questions here but see CoreAudioAndAudioUnitsTutorial first; it may point you in the right direction

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I have a problem using Apple’s Audio Extraction API in QuickTime 7. I’ve got some sample code at QuickTimeAudioExtractionExample which it would be nice to get working. Any help or advice would be warmly appreciated! - Ade 13/02/06

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How can I generate MIDI signals in my app that can be registered by GarageBand? There are apps like VMK ( http://www.versiontracker.com/dyn/moreinfo/mac/22642 ) which can do this, but I haven’t a clue where to start. –Nick, May 8, 2006

Hia Nick. You will have to do some CoreMidi work. I am not sure (I have a passing familiarity with midi), but I think you need to create a virtual endpoint in CoreMidi, then connect it in garage band, and off you go. Setting up an endpoint is a very easy task, but unless you use a simple MIDI file reader/streamer, you will have to schedule the midi notes yourself, and I believe (not sure) that this requires some real-time multithreaded programming. Take a look at snoize.com for some example code. I also recall finding more than one Cocoa-CoreMIDI bridge on the net, so google about. Also read the CoreMIDI API, it is fairly easy. –JeremyJurksztowicz

Create a client using MIDIClientCreate(), then create a virtual endpoint using MIDISourceCreate(). At this point Garage Band will show a popup because it has found a new MIDI input. To send MIDI events, create packets using MIDIPacketListInit() and MIDIPacketListAdd() and schedule them by passing the packet list and the previously created virtual endpoint to MIDIReceived(). The timestamp for a packet is one of the arguments of MIDIPacketListAdd(), so you can add packets that will be played in the future. – Maarten ter Huurne 2011-06-14