If it's just a TRRS to TRS splitter so you can plug it into a normal PC it shouldn't have any effect on the sound. The cable itself wouldn't do any harm unless it's remarkably terrible, and as for what it does to the onboard audio or soundcard in your PC, unless the mic and line out ports were designed to have separate ground for some reason, bridging them the way this would shouldn't cause any issues. If anything, using the included USB adapter (if applicable/present) would lower the sound quality a lot more than what you're doing now, as they generally have very poor DAC and amp quality compared even to what's built into your motherboard. Just keep in mind that's a generalization based on what I've seen before and doesn't necessarily hold true for the pair you're looking at.
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