I created a simple scene with an audio source and a player that can move. The main camera is a child object of the player. I set the audio source volume rolloff to "logarithmic rolloff". As far as I understand, this means that the sound should become weak very fast when the main camera moves away from the audio source. But this does not happen: as I move the player (with the camera) away from the audio source, the volume remains the same. I see, in the rolloff graph of the audio source, a line denoted by "Listener", which moves to the right as the camera moves away from the source. But still, the volume remains the same. Why?
Be sure that you have set Spatial Blend to "3D". The slider is two entries above the 3D Sound Settings collapsible section (in my version of Unity). You can also enter a value from 0 to 1, with 1 for full 3D. The slider position defaults to 0 for 2D, which will override any 3D settings you configure in the lower section.