The audioguide vendors I'm aware of include Acoustiguide in New York,
Visible Interactive in San Francisco, and Antenna in Sausalito, CA.
These three produce audioguides that provide "randomly accessed
audio."  I have heard of at least a half dozen others located in this
country and Canada.  I think Bryon Johnson's  comment regarding
economical productions is right on target.  Visitors can gain so much
using this touring method.