Yeah, your right about the wiring inside the ear piece.
Carefully pull away the plastic part that fits closest to your ear away from the stalk part.
You will either find that the wire may have become slightly un-soldered and is not getting good enough contact with the plate or that the wire has completely detached and is just barely / occasionally touching the contact.
Re-solder and fixed.
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== Update ==
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Sorry i need to elaborate.
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There will be 2 wires inside, both with their own solder points, should be obvious where they are by the locations of the solder.
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One will have become damaged, i say only one, as if both were damaged then you wouldn't get any sound at all.
Yeah, your right about the wiring inside the ear piece.
Carefully pull away the plastic part that fits closest to your ear away from the stalk part.
You will either find that the wire may have become slightly un-soldered and is not getting good enough contact with the plate or that the wire has completely detached and is just barely / occasionally touching the contact.
Re-solder and fixed.