Disclaimer: I have no experience with 13th Age, but as far as I'm aware, it's somewhat similar to D&D, even being made by lead devs from 3e and 4e. In particular, the settings I describe ahead are not system-specific, although my experience is from D&D 5e.
Additional to SSD's answer, this answer is based on recent running of Lost Mine of Phandelver (LMoP), an adventure focused for new players and DMs in D&D 5e. In the very first (admittedly short) dungeon crawling of the adventure, the publisher actually handles the problem you mention in your question and explicitly states how to deal with the combat sounds, although the system itself (D&D) has no specific rules about it. The answer will follow the format: Give general advice - exemplify with LMoP - how my players (one which is very worried about realism, similar to yours) reacted to it.
If, for some reason, an experienced player in 13th Age thinks my answer does not apply in any way, please comment.
The spoilers in answers are for text that is supposed to be only for DM's eyes running LMoP.
As SSD mentioned, one good way to make it real is just letting clear to the adventurers that it's hard to listen to anything inside that dungeon. This can be made in different ways, already mentioned in his answer - machinery, noisy rituals, rushing water or air. In the first dungeon of LMoP, it's a cave in which a river runs through. The following description is given:
Sound: The sound of water in the cave muffles noises to any creatures that aren't listening carefully.
Creatures can make a DC 15 Wisdom (Perception) check to attempt to hear activity in nearby chambers.
The second phrase is more about the adventurers than the enemies inside the dungeon - as there's no reason for them to randomly try to listen, unless they already have some information about something happening.
Just by describing the sound of running water making it harder to listen things far away my players accepted way better that the other rooms didn't instantly come to help, even being inside a cave that should propagate sound very well.
Combat Sounds are Usual
And the dwellers of the dungeon are so used to it they don't mind. This also can be done in numerous ways - infights are normal, people train inside the dungeon, weak invasions are so common that each room is supposed to handle their own fights because they're usually easy. In LMoP, the "infights are normal" strategy is used sometimes, the following being stated for one of the rooms:
Goblins in nearby caves ignore the sounds of fighting wolves, since they constantly snap and snarl at each other.
To be honest I don't know how this would get played out. My players fed the wolves and our druid talked to them, making them unwilling to fight the party. What actually happened will be expanded in the next section.
The other rooms DO listen
Well, maybe your players are right. Maybe some of the other rooms will listen and come to help. You can actually handle that by balancing the encounters assuming the adjacent room will come to help the current one.
For the previous scenario, the Goblins actually noticed that the wolves stopped making noise (because they were fed), and that was more unusual to them than the wolves making noise and killing each other.
The wolves were as friendly to the goblins as they were to the players, so the encounter from the adjacent room was moved to this room. Nothing indicating a TPK so far.
Aside from my own version with this, the "get help" method is explicitly stated in the same dungeon. In area 7, it describes:
The noise of the waterfall means that the creatures in area 8 can't hear any fighting that takes place here, and vice-versa. Therefore, as soon as the fight breaks out here, one goblin flees to area 8 to warn Klarg.
The encounter in Area 7 is easy (really easy). If the enemies succeed in getting help, though, the total encounter becomes deadly. As a note,
In the official adventure, Klarg doesn't even come to area 7 to help, instead he hides in Area 8 in order to surprise the adventurers when they get there, supposing his goblins will die to the party.
For this one, my players managed to kill the goblins fast enough so they didn't have time for getting help. As such, the encounter was the easiest they had until now.
Maybe they DID listen, but so what?
Depending on your setting and your characters (NPCs), maybe they did listen to the fighting, but aren't bothered enough to go help. Again, this has numerous justifications - "Hmm, the fight is happening in the Captain's room, he didn't ask for help, if he dies I'm promoted... Nope, I didn't listen a thing." or "Well, they are my subordinates, if they can't handle a simple invasion, I don't care that they died."
This can be further explored when the players get to this NPC, which will talk to them like "Oh, you killed our Captain. If I kill you now, I'll be getting so much honor and fame! Thank you for your services." or "Hmf, I knew that those weaklings were useless. They couldn't even handle a bunch of lowly invaders."
This is exactly what I mention in the second spoiler from the above section.
The point here is: just because they live in the same place, it doesn't mean they are all friends and willing to help each other all the time.