TiVo technical support won't help with this, because it is a playback issue. From TiVo Desktop 2.8.3 "TiVo Desktop Troubleshooting" I get:
You do not have an AC3 audio codec installed. Playback of audio from TiVo recordings from some TiVo DVRs requires an AC-3 audio codec, such as the one included in theTiVo Desktop Plus upgrade. The Home Premium and Ultimate Editions of Windows Vista include an AC-3 codec and is often included with software for playing DVDs.
I am using Windows Vista Basic. How do I best go about getting a Dolby AC-3 codec, so that audio will work on playback?