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.