In a pretty technical how-to, Engadget explains how to use your Tivo to stream music.
You'll need a few pieces of software to pull this off. Winamp will monitor your sound card and pass the audio to SHOUTcast server software. The SHOUTcast server then rebroadcasts the audio from your PC to TiVo. In theory, TiVo's Desktop software can stream MP3 broadcasts to your TiVo. In reality, TiVo's recorder isn't very fault tolerant and your TiVo server software may crash while attempting to pass networked streams. Therefore, you'll use Galleon's proxy to serve audio to the TiVo.
How-to: Stream music through your TiVo [Engadget]