The default RollOfMode is called Inverse so I think it may be using the Inverse square law for the calculation.
Combine with this to calculate decibels (for sound pressure level):
decibels = math.log(sound.Volume) * (6 / math.log(2))
or decibels = math.log(sound.PlaybackLoudness) * (6 / math.log(2)) - 54
The inverse square law for sound would be:
I = P/4πr² (I = intensity, P = sound pressure/power, denominator is sphere area)
I don’t know much if this would help you calculate sound attenuation the same way Roblox does it, but sound attenuation in an area with no reflective surfaces certainly works using the inverse square law.