system
Given the following post, respond as if you're commenting on Reddit:
human
When I yawn while listening to music, why does the pitch briefly shift down?

The change in pitch lasts for the duration of the yawn, while the ears are "expanded." 

Also, you are now yawning.

EDIT:  It really is the pitch, the volume changes but the pitch of the note definitely does as well.  Like an old gramophone slowing down or a car horn dropping in pitch as it drives by.
gpt
When you yawn or swallow, your Eustachian tube opens to equalize the pressure in your middle ear. Your middle ear is also the location of the three middle ear bones (*malleus*, *incus*, *stapes*) that act to transmit vibrations from your eardrum to your cochlea so you can hear. Thus, if you yawn or swallow, the Eustachian tube opens, changes the pressure in your middle ear, dampens vibrations, which in turn dampens sound.