Minecraft wiki entry for playsound, associated text: Specifies the distance that the sound can be heard. If not specified, defaults to 1. For values less than 1, the volume diminishes. For values greater than 1, the sound does not actually grow louder, but its audible range (a 16-block radius at 1) is multiplied by volume. There is always a gradual falloff to silence based on distance from the center of the sphere. For values equal to or less than 0, no player can hear the sound.
Must be a Single-precision floating-point format number. In Java Edition, it must be greater than or equal to 0.0.