Earpiece has not worked since purchase - sensor cables to blame?

Hi all,

I bought an iPhone 5S 64 GB from a friend of mine who used it for an entire year while it had a problem with the earpiece. The problem was that no matter whether in voice calls or using the voice memos app, no sound is played through the earpiece (yes, even over a restore). He simply hates the Apple Store and never took it in during the warranty period. I can verify that it wasn't working when he first got it because I was there that same week. It works great when used with headphones/headset and speakers. Mic works. All liquid damage indicators are white.

I took the phone to the Apple Store, where a Genius bar guy without much technical expertise tried to reproduce the issue and decided that the earpiece needed to be replaced for $60. I thought I could do better so I carefully took the phone apart. I made sure not to sever any ribbon cables and everything came apart fine. I exercised surgical precision and I know I did not stress the cables even though it was very hard. When I finally got it all apart I noticed though that the sensor cable that leads to the front facing cam and what not had some very tiny wear on the black encasing of the flex cable, right on the inner side where the sensor cable was slightly chafed by the metal plate that mounts over it. I sincerely believe that it was the underpaid Apple factory worker that put it together that damaged the flex cable. Does any data run through that sensory cable that would cause the earpiece to fail? I'm thinking that is the original issue.