You picked an easy place to start, changing sounds is a piece of cake.
The sounds you hear in Aces High are all located in a 'Sounds' folder in your Aces High directory. To find this go to 'My Computer' -> C:/ Drive (or whatever your main hard drive is) -> Program Files -> HTC -> Aces High II -> Sounds.
This 'Sounds' folder contains a whole list of folders and sounds. To change your C47 troop drop sound, go to the C47 folder and replace the file named 'drop' with whatever you want.
It is a good idea to rename your old sounds, for example 'drop old' when placing the new 'drop' sound in the folder. Leaving the renamed 'old' sound where it is will not effect the game.
To install entire sound packs the process is exactly the same. Download the the new pack, rename your existing sounds folder 'sounds old' or something and place the new 'sounds' folder in your Aces High directory. Leave the old one next to it (renamed) and that's it.