In patent application made public today Apple describes such a system for automatic generation of audio navigation tags for your iPod. The tags are generated from the metadata of your music or video files - the text information like author, song name and duration, film director or lead actor name, etc;- which comes with a song or video that you download from iTunes store.
This text information is then converted using text-to-speech software into a small audio files - audio navigation tags. The iPod computing capabilities are too small to do a good text-to speech translation, so this operation is carried out on your Mac or PC. Audio navigation tags then are attached to the songs or videos themselves and transferred to you iPod. When getting there, audio navigation tags can be stored with the songs themselves or transferred to a separate database and then synchronized with the navigation menu.