Philadelphia's Sigma Sound Studios recorded more than 200 gold and platinum records before closing its doors in 2003, eventually donating 7,000 tapes to Drexel University, including recordings by Stevie Wonder, David Bowie, Gladys Knight and hundreds of other unknown artists. Christoper Booker reports on the search for hidden gems in one of the city's most comprehensive musical archives. Stream your PBS favorites with the PBS app: https://to.pbs.org/2Jb8twG Find more from PBS NewsHour at https://www.pbs.org/newshour Subscribe to our YouTube channel: https://bit.ly/2HfsCD6 Follow us: Facebook: http://www.pbs.org/newshour Twitter: http://www.twitter.com/newshour Instagram: http://www.instagram.com/newshour Snapchat: @pbsnews Subscribe: PBS NewsHour podcasts: https://www.pbs.org/newshour/podcasts Newsletters: https://www.pbs.org/newshour/subscribe