These are the instructions from the Adobe site for CS5 / CS5.5, have you checked this setting?
– Premiere Pro > Preferences > Audio Output Mapping (Mac OS).
– In the Preferences dialog box, from the Map Output For menu, choose the driver for the device desired. By default, Premiere Pro selects Built-In (Mac OS).
You want to make sure that’s set on Built-In. If it is, then you got me puzzled. I don’t have a Mac with BMD hardware in it, so I can’t see if I can reproduce it, but on my PC, all my audio in Premiere goes through my Decklink SDI card, regardless of timeline preset. I can change it via the windows version of the settings above.