Loudspeaker does not work after screen replacement

A customer came in with a Galaxy S5 that he had replaced the display assembly on himself. Overall, it actually looked like he did a fine job of it. However he said that after he changed the screen, the loudspeaker worked once, then when he turned the phone off and back on it stopped working and has refused to ever since. He still gets sound out of the headphone jack and over Bluetooth connection. He has tried everything (and I do mean everything, I asked) short of replacing the actual speaker unit with no luck. If a new loudspeaker does not solve this problem, what could be going on here?