I'll start with Silberfuchs, because I like those questions.
1. No. If all the airflow in your face is supersonic, then you can't get a sound wave to move forward. You probably can't even make a sound because the dynamic pressure on your face from supersonic flow right at it would be quite painful. That's theory, now for truth. There is probably air that is not moving supersonic relative to you stuck in the cockpit, you should be able to get the sound to move forwards in that. Also, sound travels in metal so you could try to send a signal through the canopy frame.
2. Yes. Not as effective since it wouldn't travel forward from the train, but since you can see the lights from the sides, that will still work although it would appear as a point flash of light from "nowhere".
3. That would work, although to be truly honest to the aircraft limitations, you'd have to invite the passengers to use another of the conveniences of modern airliners. Easier would be to get the trolly dollies to leave their makeup bags behind.
Birds flying in Airplane
Final answer is the airplane won't climb.
The birds stay aloft by pushing air downwards. This is the same as airplane's although if you follow the Bernoulli argument, you have to dig deep into the physics to see that air needs to be pushed downwards. Trust me.
The downwards moving air hits the floor of the cabin and creates a force equal to the weight of the pigeons (as long as the pigeons aren't climbing or accelerating).
The effort exerted by the pigeons shows up as kinetic energy of the birds, and kinetic energy of the air inside the cabin. Eventually some of the air will be slowed, and that will create a small amount of heat (friction).
The vortices appear directly underneath, but the aircraft/bird will move forward, making some think that vortices appear behind.
If the downflowing air doesn't hit the cabin floor, then the entire weight of the birds isn't felt by the aircraft anymore (extend this to C5 with open doors, mesh cages, etc.)
There are lots of ifs ands and buts that haven't been thrown in here yet, but in the end, For every action there is an equal and opposite reaction. To get a bird up in a mass of air, some of that air must come down. QED
Matthew.