... it only seems to be one (or both) of those two files that's the problem (foobar is playing everything else fine including further audio files I have burned since those). I took the obvious step of just burning the converted wma file again and that's the problem. Foobar doesn't see it on the CD at all - no autoplay and, in fact. foobar doesn't even open.
Is that file cda or wma on the CD ?
You converted the wma to mp3 and then burned the mp3 to CD ?
Just out of curiosity ( since you seem to be done with this ), could you convert the wma to a wav file, burn to CD and see if that works. Make sure there are no issues during the conversion/decode/encode.
Instead of "wasting" a CD, "burn" to an ISO file and mount that in Windows 10. This is "built-in" to Window 10 and also an option with third party programs/apps like CyberLink and possibly Nero.
Create an ISO with the original files ( the same way you created the CD that has the issue ) and see if you have the same problem when mounting the ISO so we'll know if the simulation is the same. If it is, then try the conversion to wav and create an ISO and test that.
You can burn the ISO to the CD if you find that the wav conversion works.