Back in the days of "FPS Creator Reloaded", as I recall, the developers experimented with having NPCs say phrases upon spotting the player. Rick Vanner informed me this was dropped due to the phrases becoming too repetitive.

I have noticed this again becoming an issue in GameGuru Max as enemies repeatedly shout: "You're not allowed here!"

When developing a game called "Freedom! Do Or Die" (which you can look up on Steam), I ran into this same problem of repetition. My solution was very labour-intensive, but worth it... in essence: randomisation:

1. Identify a number of states for each NPC (perhaps 'spotted player', 'been hit', 'calling reinforcements', 'retreating'... etc.).

2. For each of the states, create around 20 variants of the relevant phrase.

3. Randomise the variants and have the NPC say the generated phrase.

It worked well for my game!

a feature that could make it more immersive would be to have hostile NPCs communicating with each other, for example warning each other of incoming explosives from the player.