Apple TV: set default to Airplay speakers?

How do I set my Airplay surround sound speaker system as the default on Apple TV? Please read before you answer.

Since I'm using a projector, my front-facing 5.1 surround speakers have to be across the room at the wall on which I'm projecting, while my AppleTV is at the back of the room connected via HDMI to the projector. So my challenge is to send audio from Apple TV to the front of the room where the speakers reside. That's basically all I do with my surround system: watch movies, netflix etc on Apple TV. So I bought a new Airport Express and connected my 5.1 surround system is via optical audio in the back of the Airport Express. But after getting it all set up and working for a session, I find that now, every night I turn on Apple TV I have no sound. So I have to repeat these steps, every time: locate and open AppleTV preferences app, navigate to the Airplay speakers settings, wait for them all to load, choose the speakers to re-connect (re-assign) my Airplay speakers to Apple TV. The same speakers I assigned the night before, and the night before that.

After assigning speakers, the system sounds gorgeous. Until the next time I return to Apple TV.

Given my across-the-room challenge, and given I chose Airplay to accomplish it, how do I assign my speakers to Apple TV—by default or permanently?