We can edit the sounds. I did that for Masters of Europe. The real problem is it's very touchy, and every time you add or edit a mod that changes the units (add, remove or change the order in the list of units), you'd have to "compile" the sounds.
And mod makers can only work on their own mod, supposing it is used completely alone.
If you want to mix several mods, the mod makers cannot know what combination you want to make, and so they cannot provide the correct sound settings. You'd have to make your own.