just looked it up, and the Creative Audigy Platinum, Audigy Platinum EX, and Extigy all have optical input, so it would be a matter of the software allowing the recording of the 5.1 signal.
IMO it's not worth jumping through all of these hoops when the TiVo has the video and audio data already on hdd. From a quick look, getting at that data on a TiVo's drive is not easy, but setting up a good good capturing / editing procedure (and buying the necessary hardware) isn't easy either.
Edit: Tornadobox is faster than me & we agree 🙂.