The latest trend in sightseeing is downloading free podcast-style audio tours to your Ipod or MP3.  This is happening all across America, from an Atlantic City boardwalk to the Williamsburg neighborhood in Brooklyn to Millennium Park in Chicago.  Accompanied by a map, the advantage of an audio tour over a more prosaic travel guidebook is the use of experts talking about subject they know best.  Take, for example, the hour-long Boston Harborwalk tour, which leads you to 17 sites from the North End to the South End (www.bostonharborwalk.com/audio_tour/downtown.html).  Mayor Menino welcomes you, soon followed by a member of the Sachem tribe discussing Native American settlement along the harbor, a trainer from the New England Aquarium talking about seals, and the owner of Hook Lobster Company telling us about his business.  A second MP3 tour offered by the city brings you to Fort Point Channel (www.bostonharborwalk.com/audio_tour/fortpoint).  You’ll check out local favorite, Lucky’s Lounge, hear about sculptor Ana Crowley’s life living in an artist-owned co-op, and see where Martin Sheen fell to his death in the Academy-Award winning, The Departed.  It’s a great way to get a workout and learn a thing or two.