Minecraft wiki entry for playsound, associated text: Specifies the volume for targets outside the sound's normal audible sphere. If a target is outside the normal sphere, the sound is instead centered some short distance (two blocks away) from the target, and this argument determines its volume. If not greater than 0, targets outside the normal audible sphere cannot hear the sound.
If not specified, defaults to 0.0.
Must be a Single-precision floating-point format number. In Java Edition, it must be between 0.0 and 1.0 (inclusive).