Remember the wind file that played back on IA? Just plop this in legions/game/client/scripts/client.cs, somewhere towards the end of the Client::joinServer function.
Code:
$AmbientWindSound = sfxCreateSource(WindSound);
$AmbientWindSound.setVolume(0.2);
$AmbientWindSound.setPitch(0.6);
$AmbientWindSound.setName(WindSoundSource);
I can't quite get it to stop and start between missions, so it'll just keep playing when looking at the mission loading screen, as well as if you exit the server. The volume is a bit too high, but you can adjust that in the code, along with the pitch.
edit: So apparently the volume code doesn't work. Perhaps it's best to leave this out, until someone figures out how to lower the volume at least (I suppose you could just make the file itself lower).