First here's the details of my setup:
MacBook Pro with Mountain Lion
Novation Impulse 49 USB MIDI keyboard connected to USB port
I bought this keyboard yesterday and it came with Ableton Live Lite 8 which I've installed and everything works fine in that.
I'm more used to GarageBand and wanted to use the Impulse with it but the audio cuts off after a few seconds. The keyboard still registers MIDI signals in GarageBand but there's no audio. The keyboard has a built-in template for GarageBand.
If I change instruments in the instrument browser the audio comes back for a few seconds and then cuts off again.
Same if I click 'Play' and then 'Stop', the audui comes back for a few seconds.
I get the same behaviour when trying to play along to any of the GarageBand piano lessons. The audio from the lesson and my keyboard both come through at first then after a few seconds my keyboard audio cuts out.
I'd welcome any suggestions as I would like to get this working reliably with GarageBand.
Thanks in advance.
After opening a support case with Novation (who were extremely helpful), they have isolated the issue as being related to the GarageBand template in the Impulse 49 keyboard.
Using the GarageBand template with the Grand Piano or Classic Organ instruments results in either incorrect or missing MIDI messages to let GarageBand know when a key has been releassed. This meant that as you were playing, the notes were just building up and never releasing, until such point that GarageBand couldn't process any more of them.
The MainStage template in the Impulse 49 works fine with GarageBand and so this is the interim solution.
Eddie from Novation has raised it as a bug with their developers so there will hopefully be an updated GarageBand template available at some point to provide a permanent fix.
Apparently, this hadn't been raised as an issue before so it may have been a result of a GarageBand update.
Hopefully this information help someone in the future.
Thanks to Edgar for his assistance and suggestions and also to Eddie at Novation for being so helpful.