@OP Have you listened to the sonic effect of various Ethernet cables or have you simply decided "in your head" that no difference exists? I'm genuinely curious to know.
I have tried, even with very high end streamers. i don't even like streams anyway, but gave it a shot.
In every case, the dealer/salesman was adjusting the volume slightly higher or lower. How do I know this? I brought a friend with me who was "on his phone" and he made sure to watch the guy like a hawk, noticed the him doing this while I was "browsing the store." Each time I returned to listen to a new song (I said I needed to clear my palate) this volume change was made.
You can go ahead and call me a liar. I'm telling you about my honest, real-world experiences. You can go ahead and try to mock me. Water off a raincoat. My beliefs are my own and I don't owe anyone appeasement.
If that clashes with what you believe about something audio related that has turned in to a form of extremism (Like some who are ultra-religious) then more power to you.
Long story short - Ethernet is a MATURE technology. When you say the sound changed, what you are most likely to reference are inconsistent volume levels and/or the difference between a poorly constructed ethernet cable with bad shielding in an otherwise heavily noise polluted environment from other electronic devices.
Aside from meeting the JDEC specification, an ethernet cable should also be well shielded. I cannot stress this enough; because our modern world is full of electronics such as Wi-Fi and other wireless technologies like bluetooth, radio frequencies going through you outlets, etc. That it just makes sense to isolate your audio equipment from these noise sources; even if they have robust circuits that should nullify the noise.

