My AirPlay setup is messed up since a few days : I have headsets plugged to an Airport Express (AE) and it used to play music synchronously with my macmini, as expected.
One day, while I did not do anything special, the macmini stopped to play music as soon as I switched on the AE headset, as if the music stream was redirected exclusively to the AE.
Yet, iTunes is properly configured in multiple sources mode :
If I uncheck the Fabrice's Airport source from iTunes and leave Computer as the only source, the problem remains : music still played exclusively by the AE ! Only switching off the AE makes the macmini emits the music again.
I tried unplug/replug the AE with no success. Next step would be to reinitialize it but I'm open to others suggestions.