A couple more things on the subject of sound. First another good source of stock sound effects. Some free content and some paid, but seems to be reasonable rates.
http://www.audiomicro.com/ I've just sourced a decent free biplane sound for my project here. next a tip for such looping ambient audio: quite often the sound will make as much sense, or sound much the same backwards as forwards, so a trick for extending the sound duration and making the loop point nice and seamless is to make a copy of the original sound, reversing it, and tacking it on the end.
Lastly, I've just noticed something really handy in the Unity editor when dealing with 3d sound (ie a sound positioned in space). If you look at any tutorials for Unity and audio you will probably have seen the nice graphic roll-off curve in the inspector (showing how your sound gets quieter with distance). I've just noticed, while looking at that while running in game mode that the line which marks the position of the 'Listener' is dynamic and moves back and forth along the curve as the sound gets nearer or further away; either as the sound source moves (in the case of my biplane) or as the listener moves through the scene. Very handy indeed! I think, by default, that the 'Listener' is at the same point as the active camera; though I'm sure that can be changed elsewhere if required.