There’s been this audio bug for a while whenever you pass through sword room/area and it mutes gunshots and footsteps. I think this audio glitch has been like this for a while. Just wanted to point it out so it can hopefully be fixed 
It’s been this way since Halo 4 on the 360, if I remember it right. I don’t think it’s getting patched anytime soon. I imagine something like that would be a very low priority since it hardly affects gameplay.