Ethernet cables are directional whether they are marked or not- at least the silver plated wire ones are. So if it sounds bad, try turning it around. I bought some silver plated wire Pangea ethernet cables about a year ago. They made my previous NUC based music server sound better. They were not marked but they were directional. It wasn't hard to tell the difference. The highs were edgy and grating one way and smooth the other way. I had the same thought at first- these silver plated wire cables sound worse than my basic cheap ethernet cables. Then I thought to reverse them and they sounded much better. I don't think it took much time to tell the difference. So I had to try out each cable both directions, and I marked them; but by the third one I could tell right away. Also, the cable closest to the server is most noticeable with the cable between modem and router the least noticeable. The Pangea are very reasonable offering silver plated Cardas wire. I later bought some AQ ethernet cables with thicker silver plating. It improved the sound but I'd say the improvement didn't match the increase in cost. I kept the AQ because cost is a secondary issue for me at this point.
Hearing a difference in ethernet cables is system dependent. The better cables make the TV picture look better but I don't hear a difference with my HT system setup. With my stereo system the difference is night and day. I have found a handful of songs that have bad mic clipping with my stereo system. I can't play those songs on my stereo, it's too grating. Play the same songs on my HT system or in my car and I can't hear the clipping at all. The detail just isn't there. I can hear guitar strings rattle in some songs on my stereo that cannot be heard in the car or on the HT system. You just don't know it's there until you hear it in a more critical system. So, careful what you wish for.