Lack of bass / audio distortion using headphone socket

I have a strange problem with an iPhone 5 I repaired. When I opened it I found the glass that had been broken on the front had severed the cable here:

http://img27.imageshack.us/img27/9793/h6...

I bought a new dock assembly which included a new earphone socket but this cable was not attached. Does anyone know what it does?

The phone is now working but when I listen to audio through the headphone socket there are problems such as severe lack of bass. It makes songs sound totally different from normal.

I have restored the iPhone to 6.1.4 so it shouldn't be a software problem, equaliser is off.

Thanks