It's because you're not hearing it with your ears. You're hearing it with your mind. This is kyber. It's what makes a lightsaber truly a lightsaber.

[ He thinks back briefly to his second time in the caverns of Illum, begging the Force for a another chance after his Master's crystal was crushed, and how lucky has was to be called by this split crystal. He didn't think he'd get that chance. ]

Kyber is... alive you could say. It's an existence that's unfathomable to someone like me or you. But what you're hearing is proof of that life. That's it's voice, it's song. And where I'm from only a very small number of people can hear it.