it might be possible to do this if the apple quicktime drivers and the blackmagic drivers support this mode of operation - i am not sure they will.
this is outside of my control.
what you need to do is use the 'audio midi setup' application. its not a system preference, its in the applications->Utilities folder.
See where it says 'Default Output', see if you can route this to the decklink card?
Quicktime 6.5 and below always route the movie audio to the default sound output.
This may or may not give you audio output from your movie.
If it doesnt then there is nothing i can do.
the computer/drivers does not support that mode of operation.
but there is another alternative if you want balanced output:
you may be able to use the g5 optical audio output ( again not tested by me...) with a convertor to give balanced audio output.
I managed to find such a convertor from m-audio, there may be other products: