"Why is it that, every time I turn on the ceiling fan, the sound on my TV set drops dramatically?"
For a test, put your head right up next to the speaker, and then turn on the fan (or have someone do it for you if you can't reach). If you don't notice any volume drop when you're directly adjacent to the speaker, then Twik is right and you'll have to relocate the TV or sound system. If you can hear the drop when you're right next to the speaker, then I'd assume either a power or RF interference. If your TV's sound is wireless, move the transmitter. If it's wired, plug the TV (or sound system if you're using a separate sound system) into a different outlet.