That sounds like a hardware problem, but you still might consider installing plasma-widget-veromix and using it rather than KMix to control your audio setup. It's especially helpful if you have multiple audio outputs like a phono jack and HDMI. After installing the package, open the panel menu by clicking the "cashew" on the right, then click Add Widgets. You'll see an entry for Veromix.
I bought a $10 USB audio device when the phono port on my laptop quit working. Plugged it into a USB port, and Linux recognized it right away. You might consider that alternative as well. BTW, it had considerably more gain than the computer audio system; make sure you turn your speakers down when you first try it out.
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