I don't know of any "official" way of doing this. It may be possible to hack the Apple TV in order to gain access to them (but doing that may wipe the content in the process). If it's the only copy of the music / videos you have then the best / only way would probably be to open the Apple TV up, take the disk out and mount it in an external caddy. That way, you should be able to copy the files straight off the hard disk...