I just had the same problem after replacing the lcd/touchscreen.
I checked the continuity of the ribbon, and that was ok.
Then i checked the continuity of the speaker, that was ok.
Then i removed the speaker from the plastic enclosure and placed it on top of the metal contact pads while playing some music - and it worked.
So it turns out that the contact legs on the speaker are not in alignment with the pads when it was installed into the plastic enclosure and that was back in the phone.
I hope that helps.
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