Hi,
This might be normal behavior if you are using the DeckLink card to generate a sync signal for the deck during capture. Please check the technical note How to sync a deck with a DeckLink card and specifically note the section named “Audio monitoring issues”. You will note further down in the technical note, it mentions the following.
If you wish to monitor audio while capturing video from a deck, then you will need to use a blackburst generator to keep your video in sync. Disable the option “Black Video Output to Deck in Capture” and make sure you choose the “auto” or “external” reference setting on your deck.
In regards to playing out sound from anything else including iTunes, open your System Preferences and click on Sound and the click the Output tab. Select DeckLink Audio and all sound, including system beeps, will be outputted through the DeckLink card.
Regards,
Luke Maslen
Blackmagic Design