I just figured it out! I just realized that I didn't fully reconnect the earpiece and front sensor assembly cable when I sealed the device back up. I opened the phone back up, pushed the connector into place, and the earpiece works fine now. :)
So for anyone reading this, before you seal your device back up, power it on, test all the functions of your phone, and make absolutely sure everything works, and that all connectors are pushed fully into place, so that you don't end up wasting a set of adhesive strips like I did.
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