Airtunes not working since updating to El Capitan

Since updating to El Capitan, I can no longer connect my 2013 macbook pro to my Airport express to use as an Airplay receiver. I've tried restarting all the devices involved but this hasn't changed anything. I can still use the Airport for music with my iPhone and iPad.

If I try to select my airport as a sound output in System preferences or on the volume control, it allows me to select it, but when I check it again, it has reverted to 'internal speakers'.

Strangely, I can still use the macbook for AirTunes if I select the airport as a sound output in iTunes itself - but this only works for this program. Spotify, for example, can't see the Airport or allow me to select it.

Anybody any suggestions?