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Quicktime Player issue with OS 10.8.x and Avid
There is currently a Quicktime Player incompatibility between MacOS 10.8.x and Avid audio hardware. It is not currently possible to play Quicktime Player audio through Avid audio hardware via the Core Audio driver.
You can try VLC Media Player as a substitute for Quicktime Player.
From the Pro Tools 10.3.3 read me file:
QuickTime Player is not currently compatible with Avid Audio Interfaces on Mac OS X 10.8 (PTSW-163965)
QuickTime Player does not currently support Avid audio devices as output devices with Mac OS X 10.8. Audio will not pass through and playback will not start. Please use an alternate audio interface for QuickTime Player, or use another software solution for playing back QuickTime files with Avid audio devices as a workaround for this issue.