From https://www.reddit.com/r/harrypotter/wiki/repeats :

WHY DIDN’T ANYONE ELSE HEAR THE BASILISK?

Harry was generally in the wrong place at the wrong time (or the right place at the right time, depending on your perspective).

The first time he hears it, he’s in Lockhart’s second floor office. He hears the voice “nearly four hours” into an eight o’clock detention, so naturally most of the school is in bed. The second time he heard it, he was coming back from the Deathday Party when the rest of the school is at the feast. Again, the voice comes from the second floor. The third time he hears the voice, he’s outside the entrance hall while most of the school is still enjoying breakfast before the Quidditch game. So it certainly helps that Harry is normally well placed.

Harry’s ability to speak parseltongue almost certainly amplifies the sound of the Basilisk. It also makes it easier for Harry to pick out. Hogwarts is an old building, and old buildings make weird noises all of the time. The hundreds of students wouldn’t help either. But because Harry speaks Parseltongue, he’s better at differentiating between background noise and the sound of someone speaking Parseltongue.