Capture card with zoom

I have a capture card bringing live and recorded video in from a dslr.  I can select it for both audio and video, and the video comes in just fine.  But no audio comes through.  
1st, am I missing anything beyond selecting that capture card as the audio input? And 2nd, the compatible campfire cards that zoom recommends for Mac were all quite expensive ($400-some). Has anyone had luck with a less expensive kind. For both audio and video?