When adding a video capture device or camera that provides both video and audio, there should be a pulldown menu in the properties of that device to specify how to handle audio. There should be a checkbox for audio, and a pulldown asking which device to use for audio-- from which you just choose the name of the capture card.
If you try and use the device as a camera in QuickTime or Skype, does it provide audio then? If not, then it might be a driver issue.