Source:setAttenuationDistances
Sets the reference and maximum attenuation distances of the Source. The parameters, combined with the current DistanceModel, affect how the Source's volume attenuates based on distance.

Distance attenuation is only applicable to Sources based on mono (rather than stereo) audio.
Source:setAttenuationDistances( ref, max )
ref number The new reference attenuation distance. If the current DistanceModel is clamped, this is the minimum attenuation distance.
max number The new maximum attenuation distance.