Built in Mic not working on oovoo/ other video calls? help?

I own a MacbookPro, not bought to long ago. Macbooks seem to have a built in mic. But Im wondering if I have to buy a separate mic for video chatting, etc. For example when I use Oovoo I can hear everyone else, yet it wont pic up my mic. But when im not using oovoo or some video chat app, and just taking a video, my microphone works perfectly fine. Do I need to buy a separate mic for video chats like this, or is there some way to fix the problem?