Htc one m7 - Rip a flex cable. What is it?

I open the phone to replace the camera. After I open the phone, I saw that I rip a flex cable that is attached to the top mother board. The location of the flex: with the screen facing down, it's on the top most on the left, just above the volume buttons.

It's thin and got a long u shape loop in it.

I turn back on the phone, everything seems to work fine.

But still would like to know what is it.

btw, some suggest its for the speaker, i checked, both my top and bottom speakers are working.

Thanks