Have not seen this before, but have you tried the usual?
Repair permissions (in Disk Utility)
Trashing preferences (spotlight search for garageband plist file - not at my mac just now so I don't know exactly what it is called)
I also remember some problems a while back which required CoreAudio to be re-initialized. However, as this was done by launching Garageband this method may not help, but there might be another way of re-initializing it.