How can I add audio to resources on a flip card container?

You can add audio to objects too. Right click on object and at bottom above Apply Effect is Audio. Select that and you can add/remove  even edit audio there. (I'm using Classic)

Please it is very important to know which version you are using, full version number. At this moment there are two very different versions used everywhere. You find the version number under Help, About Captivate if you are Windows. Just some general knowledge at this moment.

Captivate can have multiple types of audio. You have indeed Slide audio which is synchronized with the slide. But normally you can add audio to objects, which will play when the object appears. Moreover there is an action 'Play audio' which can be triggered by any event, and you point to the audio clip you want to play. There is of course also background audio which is added to the project.