lost external speakers in mountain lion

With Mountain Lion you should be able to output iMac sound to the airport speakers. But with me, it only works after rebooting the iMac. Currently, the airport speakers are an option in the output list on the sound preferences, but whenever I chosse them it automatically goes back to the internal speakers. I cannot choose the external speakers.

Stopping coreaudio process works, but is a hassle to do each time.

Do others have this same issue and if so, how have you solved it?