Hello. In my opinion it's not volume flex damaged. But ive got questions for You :
1) Does your phone getting lag when trying to rec a video, or trying to watch any video with sound on YT
2) Does Phone lag while some1 calling you, when you have sounds turn on , and does he works properly when you have sounds off ?
When phone works properly on headphones You could have damaged sound IC, it's very expensive in repair, and very dangerous, beacause its "under" screen connections on Mainboard.