stefkeB

You can link Garageband from within Live, for both MIDI and Audio. 

1. install Soundflower from Cycling '74, reboot, run Soundflowerbed
2. Start Live as your master application to route everything.
3. Start Garageband as a second app, only for processing MIDI and audio from Live.

 There are different scenario's, but what worked for me: 

• You can send the MIDI from a Midi Track in Live to the IAC MIDI Driver. When you play notes in Live (or have a pattern), they can be received inside Garageband. There has to be a virtual instrument and it has to be monitoring (in record mode). No need to launch the GB song, if you don't need the loops from GB.
• You can use Soundflowerbed to route audio between applications. I've set Garageband to output to Soundflower 16ch and receive input from the same Soundflower 16ch virtual driver. It seems that the output from Garageband is sent to channels 1 & 2 only. Not sure if you can change this. This is what is being sent to Live.
• I've also sent some audio from Live (loops or live input) to different channels (e.g. 9 and 10 -- both in mono, for two electric guitars). In Garageband I created two tracks for real instruments to monitor live input from the same two channels. Now you can apply Garageband effects, e.g. the guitar presets or whatever. The processed signal from Garageband will return through channels 1 & 2.
• Inside Live I receive the combined Garageband output through channels 1 & 2, which I can then mix with the rest of the Live signals. You can add effects.
• To actually hear something, I've routed Ch 15+16 from Soundflower to the system audio (or to the external audio card) and used these channels as Master Out.

 PROBLEMS: 

• When anything is wrong, you'll hear nothing or you can create a real feedback, which might destroy your monitor (or your ears).
• You can only monitor one single virtual instrument inside Garageband. To have more, you have to record them as audio loops (in Live or in Garageband).
• You can monitor at most 8 real instruments in Garageband.
• I've had a noticable latency (Powerbook G4, 1.6 GHz), so I've had to use delay compensation in Live, to move the tracks ahead of time (about 50ms). Not a huge problem for recorded audio, but problematic in "live" recording.
• There seems to be only one stereo pair that returns from Garageband, so you have to do all processing either in Garageband or on the combined signal.