I used to think that there were two different messages:
"Ace under attack" and "Base under attack"
I thought that "ace" was yourself and meant aircraft approaching you, and that "base" meant that a/c were approaching the base you'd taken off from.
I don't know how many times when I first started that I'd be cruizin' around and hear what I thought was "ACE UNDER ATTACK" and start turning right and left looking for the enemy aircraft.
But, since I switched to Mitsu's soundpack, I'd forgotten about that. Now, IIRC, it's been replaced by "scramble, scramble!!"