The Tardis sound is the result of a process, so is perfectly protected. It's not a sound that exists naturally - it's a construction. So if you replicate it, so it sounds like the Tardis, and in the minds of the listener, is obviously the Tardis - then don't expect a Judge to find in your favour if you get sued. The process is fairly well documented - piano string scraped and reversed and then combined. Each new recording of it is subtly different, but all sound like the Tardis. Don't forget that copyright does not have to be 'registered', just proven.