Why doesn't my desktop work with Line-In Headphones?

I bought a pair of KOSS UR22i headphones, it has Line-in, and my desktop computer(Gateway SX2110G-UW318) says it is working properly, but when I use it, it never records my voice, I do press the button, I have an audio driver(Realtek), but it never wants to work, I know it isn't just these headphones, I have taken back these headphones & replaced then 8 times, and also tried different line-ins, but they NEVER work. Why won't they work? And if you know, how can I fix this?

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