The adapter you supplied the link for, is just a tiny piece of optical fiber in two different ends of molded plastic, which will not work for you.  An actual converter from SPDIF coaxial to TOSlink optical would have electronics inside, and a power supply.  Try searching on amazon for “spdif coaxial to optical”.  In theory it’s a no-brainer conversion so the cheapo units should work just fine.  Crutchfield didn’t seem to have any name-brand converters.  An alternative would be to use an old Sonos Connect, which had a TOSlink optical output, instead of the Port.  If you get a Connect Gen. 2, it will even run with S2 app.  (You can check if you’re not sure if it’s Connect Gen.1 or Gen.2: https://en.community.sonos.com/components-228996/connect-legacy-vs-connect-modern-6836127 )

Thank you for the info. I actually bought a cheap converter off Amazon. Worked fine at first. But now I’m getting all kinds of cracking and popping when I use it. No clue why. 

Probably because it’s cheap junk.  They all are, with whatever multi-letter brand names these Chinese spoofers keep coming up with, to continually rotate away from the previous brand and its reviews.  Just return it and buy another one that actually looks different (external swaps of ports typically mean a different circuit board design behind the panel). 

There are other possibilities than cheap junk, but they are less likely:

-- Snap, crackle, & pop aren’t usually from bandwidth/signal drop-outs in the music stream, which usually just causes silence.  But on the outside chance, you could hook up something to the Sonos Port’s analog outputs at the same time, and see if it mysteriously mutes during those same crackly stretches, in which case it isn’t the converter’s fault.

-- The KEF LS50W aren’t 100% perfect either, but their QC on Chinese assembly is way better than the  $15-20 gadgets.  If you suspect the noise is coming from the amp inside the speaker, it’s easy to switch to a different non-Sonos source with the app.  If it’s heat-related the crackle should continue.  I doubt this too, but at least it’s easy to test.

-- Old coax cables and even TOSlink cables can shift around, giving you a flaky connection.  But again this usually doesn’t cause crackling, rather silence stretching out longer and longer.  But you never know what the converter will do with “bad bits”.  My experience with cables is the opposite of most people’s: I can use any old cheap RCA audio cable for the SPDIF coax, but the optical cables never seem to pass enough light across their butt connection.  Pull all the cables out, blow out the optical connectors, and re-seat them, maybe giving the coax connector little tiny twists left&right as it seats.