AudioViator is an excellent collection of audio tours of cities and landmarks all over the globe. All of the tours that I previewed are Creative Commons licensed and available to download for free. You can search for audio tours by keyword or use the map to browse for free audio tours. I used the map to find audio tours of Boston's Freedom Trail and Mount Rushmore.
Applications for Education
If you're taking your students on a field trip, take a look at AudioViator to find an audio tour that your students can use while on that field trip or in preparation for it. You could also use the audio tour files as part of a Google Earth tour. For example, you could take the Mount Rushmore audio file then have students create a visual tour in Google Earth to match the audio tour.