I managed to fix this but it was a pain. I reopened my device and followed the screen repair walkthrough up to removing the speaker from the screen. At this point, I tested just the speaker not attached to the screen and it was very loud and clear, which meant to me that there was an issue with the speaker grill. Below is what I did to fix it.
Before attaching the sensor module to your replacement screen, ensure there is nothing blocking the earpiece screen on the new screen part. Mine had a mfg defect or something where there was a black foam like film on the outside of the speaker grill that I couldn’t remove with tape or sticky putty. I had to blow through the back of the grill with a computer duster compressed air can. Otherwise the call volume via the earpiece is extremely low, even when turned to max volume.
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