The problem might not be the earpiece speaker! I replaced mine and had no improvement in quality. After reviewing forums, I suspected the earpiece screen was dirty/plugged. I disassembled the phone again, removed the earpiece speaker, carefully scraped and poked at the earpiece screen until a flashlight revealed the holes were clear. After a quick reassembly, playback of a voicemail verified full volume has been restored. Since you’re in there anyway, whether the problem is the screen or the speaker, clean the screen and replace the speaker.
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