A TourGuide is GPX, MP3 and JPEG files bundled. Garmin POI Loader will
take all the necessary files and create what's called a GPI file.
Doug Adomatis from TravelByGPS.com already experimented with TourGuide. Read this article on his site: http://www.travelbygps.com/TourGuide.php
As a moderator of the Yahoo! Map Authors Group he provides some more details about creating TourGuides in the following thread: http://tech.groups.yahoo.com/group/map_authors/message/6473
If you would like to know more about TourGuides or want to play with it yourself, the above articles are a "must read" for you.