It had been frustrating me for weeks now, not being able to select my AirPort Express as the audio output for my MacBook Air (wanting to stream Spotify to my speakers). The selecter would simply keep jumping back to ‘Internal Speakers’. My initial Google search on the matter told me that I was not alone with this problem (it’s something that has popped up since the advent of Lion and Mountain Lion apparently), but the only solution I could find that worked was to restart the Mac which is rather annoying to say the least.
But today I found a permanent answer on this page.
Basically, you need to disable IPv6 on your Mac and this is how you do that:
1. Open Terminal (found in your Applications>Utilities folder).
2. Type the following:
networksetup -setv6off Wi-Fi (<- if you're connected via WiFi)
networksetup -setv6off Ethernet (<- if you're connected via Ethernet i.e. wired)
3. Press Enter.
4. If prompted for your password, enter it.
And voilà - you can stream through Airplay again (well, I hope you can, I could).
/// UPDATE 1 DAY LATER ///
Damn.... just re-opened my MacBook after closing the lid for sleep for a few hours and the problem is back. Even typing the above in the Terminal doesn't work.
I have decided to go back to using Airfoil…. that works at least.