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.
An alternative to the Iopener or heat gun is to heat water to EXACTLY 80 degrees F (cooking thermometer is great for this) and pour into a reinforced ziplock type bag. Leave the bag on the screen, but be sure that none of the hot water rests on the zipper portion, as the water is hot enough to soften that seal and leak out on to your device! This meat had is my personal favorite as the heat is distributed very equally and precisely as compared to a heat gun, but is more accessible than the Iopener.
When reassembling or placing the whole sensor module on a replacement screen, make sure to gently rock the speaker back into place. There is a rubber gasket lip around the speaker that must be seated all of the way against the screen and speaker grill to function properly. You should feel a gentle click when is goes in. If is not seated correctly it still easily can be screwed in making it difficult to tell if the speaker is in correctly.
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.