Hummba.com is a website that offers audio guides in iPhone and Android apps for free for several cities around from North America like New York to San Francisco to Washington D.C. Guides for European cities include staples like Barcelona to London to Rome for example. Hummba.com also covers towns in Africa like Johannesburg to Dundee as well as selected cities in Oceania like Wellington or Dunedin. Each guide is available to be downloaded to your smartphone or computer. Each guide includes a detailed history of the sight you seeking as well as map support for easy directions and traveling. The guides on Hummba.com also include valuable information like telephone numbers, address, hours of operation and official website and email information for the location, whether the sight is a museum, building or a theatre. Each of the audio guide tours are published by Tourism Radio. On Hummba.com you can use the search tool and download an individual audio guide for a specific sight that you wish to visit. Download: Hummba for Android and Hummba for iPhone.
Or, see their explainer video here.