Thunderbolt Display does not auto-select audio

I've read a few historic posts regarding this but I can't fix the problem.

Until today, when I plugged in my thunderbolt display to macbook pro retina 10.8.3, the audio would automatically rediredt to the display speakers, now it doesn't.

I now have to manually select (Display Audio - USB) on the Output pane of the sound settings panel.

As I said, and have read from many others, this used to work but today doesn't. I've tried the 'power down display, unplug TB, reboot MB and then power up' but it's still the same.

What on earth has changed as from today to stop this working?

Many thanks.