There already is an audio angle of attack indicator and it's in just about every fixed wing airplane. It's the stall warning. If you're approaching the critical angle of attack, it kicks in. Even better, it works no matter your weight, balance, wing loading, airspeed or configuration. Is is as effective or complete as a nifty visual indicator that displays in all regimes of the flight envelope? No, but it does tell you what's most important - when you're running out of angle. 'Member, stalls aren't a function of airspeed, just AoA. Go slow enough and there isn't an AoA that won't stall you, but it's still angle of attack.
I would imagine also that a visual angle of attack indicator would also introduce an unnecessary incentive to keep one's eyes on the panel.
Or I could be talking out of my ass. I haven't slept for thirty hours.