iTunes will not play
Running iTunes 12.3.1.23 under OSX 10.11.1 El Capitan, on an iMac. All playlists are intact, but when double-clicking an item, the small loudspeaker icon turns active for about 1 second, then turns passive; i.e., no sound output. Selecting a station under Internet Radio, however, works fine. Have reauthorised and then re-authorized, to no effect. Further, some of the playlists are rendered subdued, while others are rendered normally.
Any suggestions as to what has gone wrong, and how to remedy the situation?
iMac (21.5-inch, Late 2013), OS X El Capitan (10.11.1)