Hello Lou, 

Apart form having a case open with Digidesign I have opened a thread in their forums where you can follow the issue:

The people from digidesign's technical support have argued that they could reproduce the issue with other applications such as Ableton Live, so its an issue with the drivers. It does not seem to happen with Audacity, so it seems related to aggregating. It only affects monitoring.  The only solution that Digidesign is currently able to offer was to suggest that I buy an external soundcard. Their sales department committed to offer a discount of 25 % on an Mbox as a courtesy because the configuration with the built in i/o is currently in their supported device list.

hope it helps

On 02.02.2012, at 17:31, Lou Judson wrote:

> Can you detail the circumstances? I almost always use auduo interfaces, but need to be aware if this is not related to whatever software you are using to record...
> Thanks,
> <L>
> Lou Judson
> Intuitive Audio
> 415-883-2689
> On Feb 1, 2012, at 12:04 PM, Gregorio Garcia Karman wrote:
>> Be aware of toslink connections to the mac on 10.6 and 10.7 I have been having serious issues using monitoring with this setup. There seems to be a bug in OSX syncing to Toslink and using aggregate devices. Audio on the headphone aoutput will get progrssively distorted and fail completely after a few minutes of recording. Neither protools 9 nor 10 will work with this setup. Could reproduce the same problem with different generations of mac minis and laptops.
>> cheers
>> Gregorio Garcia Karman
>> [log in to unmask]