What do they say when they aren't in a group? Do they just say "Moving" or "Under attack"?
I do worry that could get annoying, especially if someone clicks multiple times, which isn't super likely, but in AIWC I've done that a few times. If they kept saying "Moving" they would repeat themselves a lot and be fairly annoying.
Two things here: I would imagine that it would be modular. Most well planned strategy games let you tweak what your units notify you about. EG: Only voice notify under attack if in group. Or never voice notify under attack. Secondly In addition, you havea 10 second cooldown between the same phrase or between any phrase. The same selected group (or maybe globally) only say "moving" every 10 seconds, even if you clicked multiple times. The rest of the lines don't get said.
I have seen games that give you a huge amount of control over unit notifications as well, the conditions as well as the timings.
EDIT: I'd like to take this opportunity to further shill home worlds sound design.
You can strategically use light radio static type effects to make the sounds of unit lines less sharp, so that they are easier on the ears and less distracting.
Sorry, to answer your first question: Alternatively you could have them just notify you of the sector they are in, instead of that they are under attack. "Sector 5, under attack" for when taking damage. Again this should be an option that can be toggled to voice or notification/beep only. The bloat of number of lines required is kept to a minimum, moreso if you use ARMA style sectional dialogue.
Sector (5) under attack.
OR when sector 50 is under attack.
Sector (Five, Zero) is under attack. Or Group (five, zero) is under attack. All you have to record is the main phrase (sector under attack) and the numbers 1-9, and you have infinite sector and unit dialogue that doesn't take up space or take too long to record.