Headphone splitter is defaulting to surround sound

I purchased a headphone splitter for my vacation so that my wife and I could watch a movie on the plane with two sets of headphones, but it seems that the computer thinks that the splitter is a surround sound setup, because the audio is not playing equally on both sets of headphones (basically, one of us can hear everything, the other can only hear music and sound effects, and the dialogue is severely muted). I've gone through every setting I can find both in DVD Player and the System Preferences, and can't figure out how to switch it back to stereo instead of surround sound.

Do I just need to buy a different splitter, or is there a hidden setting somewhere that I haven't found?