While the LCDi3/i4 and iSINE's two-pin connectors aren't proprietary and many standard ones will work, we did throw in a small twist that's important to note:
You'll notice a groove on the connector of the cable (under the L and R on the iSINE cable), and a corresponding tab inside the headphone connector. This is to ensure correct polarity. When using any aftermarket connector/cable, it's important to make sure the connector fits within this tab so it doesn't damage the connector inside the headphones, as this would not be covered by warranty. The recommended dimensions for connectors without this groove is 2.5mm x 5mm, and the pins are .75mm in diameter, but .78mm will also work.
Here's a diagram of the pin assignment as you look at the cable connectors with the lettering or groove facing you:
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Here's a photo for reference.