The reflection should look the same with every single pulse of light, unless something affects it, which in this case would be the vibrations of sound.

So it should pick up sound along the entire length of cable, which would return a lot of ambient noise if you're trying to hear inside someone's house from all the way back at a utility company.

I don't know how good their audio filtering abilities are, but it seems like wherever you take measurements can't be too far from what you want to hear.