call of duty 4 voicechat

I've been playing call of duty 4 multiplayer online on my macbook. Can i use voicechat by just using the built in microphone for my macbook instead of getting a headset? also, do i need to download anything to do this?

You can, but you're not going to want to do that. The speakers will be on, which then means your microphone is going to pick up everything that comes out of your speakers and you'll get really really bad feedback and echo happening.