Why does listening to people give me a headache?

I can listen to music, machines, and other loud noises for a long time just fine, but when I have to listen to people talk for long periods of time I wind up with a headache. It doesn’t seem to matter what the lecture or conversation is about, even enjoyable ones seem to set off my head. Hell, I tried listening to an audio book last night and it felt like it was going to kill me.

What could be causing this? Why is it only human speech, and is there anything I can do or try to maybe get it to stop?