Sound coming out of TB dock and MacBook Pro simultaneously

Hi all,

I recently purchased an OWC Thunderbolt 2 dock to use with my retina MacBook Pro (10.10.5).

Everything is working beautifully but there's a small bug that's puzzling me ... having selected 'USB audio CODEC' as the sound output in the Sound pane of System Preferences, the majority of the computer's sounds are coming through the headphone port on the dock, but some sounds still insist on coming out of the speakers on the MacBook, things like new mail sounds (coming from Apple Mail), the sound the Finder makes when deleting files, etc ...

Has anyone heard of an issue like this? Could it be a firmware problem with the dock? I thought it was impossible for sounds to come out of more than one output device at a time?

Many thanks for your help 🙂

David