If you are using Premiere and the BM card, check your player settings in the preferences. Make sure you are using the BM player. Also you need to use the audio output of the BM card or the audio and video will be out of sync.
From the Decklink Manual
Blackmagic audio hardware is automatically chosen when a new project is created and the capture format
is set to Blackmagic Capture. Go to Preferences > Audio Hardware and verify that the Default Device is set
to Blackmagic Audio.
You may also wish to visit Preferences > Audio Output Mapping to select the mix of the multi-channel audio.
Set Map Output for to Blackmagic Audio
No matter what hardware card you are using (Aja, BM, Matrox) it will not be a perfect marriage with CS-5 or CS-5.5. That's a known fact. Adobe is aware and there have been some comments that suggest that CS-6 will be much better at integrating third party I/O - we'll see.