Sidenote: before checking or replacing the E51 earpiece, check the plastic net that covers the earpiece.
If it’s really dirty, it may block the sound coming from the earpiece in a manner you won’t believe. I solved taking out my steel cover (just the cover, not the electronics) and washing it in hot water and soap, and gently scrubbing it with a light brush. I dried it before pluggin it again the sound was normal again.
