I'm not sure I understand the question completely, so I'll answer in two ways:
1) if you're using the narration feature in Camtasia Studio, any existing media on the timeline will play back as you narrate. If you have headphones plugged in, it will play through the headphones. 2) if you want to hear your voice through your headphones as you record narration: no, you cannot. If this is what you're trying to do, can you tell me how it would help you with the project you're working on? I'd love to know more.