No absolute value of airspeed

Without taking the absolute value, we get the correct value on ground around 0 instead of 4m/s. Since the connection of the tubes is checked at calibration, I don't see a problem of this. And if a user still has it wrong and blows into the tube he will see a minus value on the HUD. That should get him alarmed that maybe something is wrong.

I also corrected some strange audio messages of the calibration.