Currently, when a zombie notices you, it triggers an alert sound. But that alert sound doesn't have any proximity to it. Whether the zombie is directly in front of the player or 30 blocks away, the sound always plays inside of the player.

More on this from this user, by the way.

More on this from this user, by the way.

Even if the audio sounding has converted to mono, apparently the sound source is coming from the player entity itself, so you'll hear it in (almost) full volume despite the targeting mob being far away.