You might be better able to find music that fits particular historical needs (like what your playwright might have been listening to, etc.) if you first know what music was popular during a particular time/period.
Oxford Music Online * contains several important resources including New Grove Dictionary of Music and Musicians. Use this site to find country overviews, time period overviews, biographies of composers and performers, and information about some specific pieces of music.
AllMusic Themes provides a listing of popular music (so it will generally have lyrics, which you're limited in how many songs like this you can use) that match certain themes. Some of the listed themes are Adventure, Crime, Victory, Seduction, etc.
Naxos Music Library * allows you to listen to almost 115,000 streaming albums. Many different record labels, including the prestigious Sony Classical, reside here. While mainly focused on Western art music, a decent selection of world music can be found here as well.
Smithsonian Global Sound * is known for providing recordings from the Smithsonian Folkways Recordings labels. Music found in this database represents traditional cultures across the world from Armenia to Zimbabwe.
Jazz Music Library * has almost 1,000,000 tracks (that's over 65,000 albums if you're counting) of jazz, swing, bop, dixieland, etc.