Originally Posted By: hybrid8
Some people have also recommended Mac OS X's "Audio Midi Setup" utility. But reports seem to indicate that the required "Through" check-boxes are now disabled for most people, which I've verified on my own machine.


That was first thing I thought of, in fact I once had this inadvertently set and wondered why my iBook was howling round.

Audio MIDI Setup worked on my MBP in the Tiger days however I'm now running Leopard and that 'Thru' checkbox is now disabled too.
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Cheers,

Andy M