The exhibition archives for the Metropolitan Museum of Art go back to 2000 [1999, actually, as of March 2011]. While not all archived exhibits have accompanying online media, most of them do. The Met has a generous and clear "terms and conditions" policy so that I can, for instance, show this sketch of Franz Liszt by Ingres
Franz Liszt, 1839 Graphite with white highlights on paper; 12 1/8 x 9 in. (30.9 x 22.8 cm)
Richard Wagner Stiftung, Bayreuth
Other archived exhibits have audio files as well as images. For example, Richard Avedon: Portraits has sound for some of the photographs along with the transcripts.
The Met is one art museum which takes its online presence seriously. There are comprehensive exhibits in many cases, and there's no searching around for image files, like there is with sites which provide only podcasts of the audio which a viewer would hear if she or he were in the art gallery. The Met's podcasts allow you to subscribe, listen on site, download episodes, and read the transcripts.