When the playsound command is executed, the system checks if the player is in range, then plays the sound at full volume, regardless of distance. If the sound begins playing and you fly out of range it still plays at full volume. If the sound starts playing and you move into range, it will not fade in, even if the command was set to play @a. The most recent version I played before this was 1.12.2, where it is possible to hear a sound fade in after it has already started, and fade out when you fly away from it.