Why is it that when I yawn (especially when listening to music via headphones) I notice a distinct lowering in pitch?
Your ears are probably popping. Atmosphereic pressure changes can cause an imbalance of air pressure between the outer and inner ear. That can be equalized through air entering or exiting via the eustachian tube. It is kind of a tight fit, so often the tube has to be worked back and forth a bit let air in and out. Yawning and chewing can jostle it in the right way to let air through.
Once the pressures have equalized the formerly stretched ear drum relaxes and a wider range of sound can be heard more clearly.