How can different CAT5/6 cables affect sound.

Not every streamer accepts sfp. And even if in case of streamers featuring both fiber and rj45 inputs it highly depends on a user preference for sound, implementation of the sfp and the rest of the system and room acoustics. Remember it’s all about synergy. You can degrade your system performance using fiber optic converters simply by injecting noise from SMPS units back into AC line your components are running from. Or having all this network garbage in close proximity to components and cables will impact the overall sound by the EMI radiating from these modules.
Too many variables here to make blanket statements @devinplombier 

There’s a difference between Ethernet cables in how they affect the sound. I don’t have experience with $5,000 or $8,000 Ethernet cables (and I doubt you do as well) but from what I’ve tried I was able to pick the ones that sounded best to me. I will note that none of them sounded horrible.
All Ethernet cables are equal but some Ethernet cables are more equal than others. 

@audphile1 there are indeed many variables and your points are well taken, but they don't necessarily contradict mine. 

Best practices are to run dedicated AC lines to one's system, and to keep network / computer gear in a room / closet separate from one's listening room. This way no worries about EMI/RFI interference, whether via physical proximity or via AC mains contamination.

The SFP cable that feeds one's streamer should originate from a SFP switch stashed away in said separate room / closet, and terminate at a converter fitted with a linear power supply.

All Ethernet cables are equal but some Ethernet cables are more equal than others.

Yes, that would be SFP ones! 😃

I don’t have experience with $5,000 or $8,000 Ethernet cables (and I doubt you do as well) (sic)

Correct, and here's hoping I'm always busy enough that auditioning Ethernet cables never starts looking like a good use of my time.

Happy listening!

Happy listening to you as well! 

"How can different CAT5/6 cables affect sound"

if FIFO of network stream receiving device is not overfilled, there is no impact. RFI is less of the problems, if all cables are certified. 

I have been setting up my new T8 streamer.  I was playing a work streamed from Presto and needed to re-route the Cat-5 cable.  When I unplugged the cable from the T8 - nothing happened; the music continued to play for about 30 seconds.

Since the link between the switch and the streamer uses TCP/IP and so is error free, given a competently designed streamer, and the timing is generated by the streamer's clock, this suggests that an SQ changes with cable changes are not data related but are caused by, loosely speaking, noise, picked up by the cable.

This suggests an experiment for those who hear hear SQ differences.  Connect the streamer using the WORST cable that you have.  Start playing, noting the SQ deficit vs your best - and unplug the cable.  Do the problems go away?  

Since the issue is posited as isolation - using optical connections e.g. - no cable should be the ultimate isolation.