AirPlay switches to internal speakers
Streaming music/sound from any source (including Spotify) worked fine after I upgraded to Mountain Lion half a year ago. About two weeks ago, it stopped working. When I select Airplay when Spotify is playing, there is no output for about 10 seconds, and then it switches to my internal speakers. Even when I close all apps, I can watch this happening from System Preferences - Music. When I select Airplay there without any music playing, it switches back to internal speakers after about 15 seconds. Timing-wise it may have coincided with upgrading the firm ware on my Airport Express.
Hope anyone recognises this problem, It's very frustrating, since I cannot play music over my speakers anymore.
MacBook Pro, OS X Mountain Lion (10.8.4), Airport Express