we cannot manage the never ending alert sound (siren), imagine a world full of sounds -- FPS hit ? -- . IMO this request is not a priority despite it is an interesting one. Moreover if we are going to hear wind blowing I want wind added in MA too ! 
You can lower the siren sound in game in the prefrences/sounds.
You can also make a recording of nothing and replace the airraid.wav file Or replace it with birds chirping if you like. but I suspect for the length of the loop allowed that would quickly get just as old and annoying as the air raid siren
Sure the could be included, but they wouldn't do anything. Sound packs only replace default sounds in the game, you can't replace sounds that don't exist.
Sounds are all user side.
And while all these suggestions sound nice (no pun intended). I doubt your going ot see them make it into the game anytime soon.
Custom sounds can have an effect on how well the game may run. Can you imagine what a bunch of new sounds would do?
And besides. The idea is to go out of the tower and play the game. If your in a plane or tank, the one thing you normally dont hear.
Are purdy sounding birdies