DVD dialog wont't play through headphones... why?

So Santa gave me a whole bunch of DVDs this year and I’m trying to enjoy them while the girlfriend sleeps only one thin wall away from my computer’s speakers. The volume control on these speakers has a headphone jack on it, which I am duly plugged into. And while MP3s, AVIs, games, et cetera will all render sound just fine, DVD wants to be all picky on me. Music comes through no problem. Sounds come though. But the dialog is missing. I have no idea why this is but it has happened on various computer configurations.

Specs:
Windows XP SP2
Windows Media Player 11 (Tried MPC as well but same deal)
Integrated sound

Any idea why all other sound works through headphones but not DVD dialog? I have a feeling I’ve run up against a DRM issue- my analog headphones could conceivably be a recording device and this is the DVD equivalent of Macrovision. That’s the only thing I’ve got… I hope I’m wrong and it’s just an obscure audio channel setting.

It definitely seems to be copy protection as I installed AnyDVD and that fixed the problem.