Newborn hearing?

I talk to my son every day and out two days ago my apartment's fire alarm went off and it's extremely loud and my son was sleeping he had no reaction until the alarm he didn't flinch she didn't cry he didn't wake up nothing ever since then I've noticed that when I talk to him or make a loud noise he doesn't get scared he doesn't look in the direction that the noises coming from the most babies do when he was born the nurse did tell me that most babies don't pass the hearing test the first time which he didn't but since the fire alarm incident happened I've been very worried that he might have a hearing loss and I don't know what to do. The nurse also told me that I shouldn't be so worried because at birth some babies do get fluid in their ears and over time it will come out but that could be a reason why he is in reacting to noise . So I have been worrying about this ever since. Has this happened to any mothers what advice do you have for me?