Sorry. I wanted to flash the XXUFNB9 LTE modem because of ignorance on my part.
My experience with custom ROMs is with just two other phones and they were not Samsung. My adjusted mental model of how this works was incomplete and I did not understand the big picture. I saw that there's a Optimized LineageOS 14.1 ROM, installed it with Heimdall, it didn't work with my carrier, figured out I needed to upgrade the modem, settled on I9505XXUHOE2 (again, because I didn't know any better), tried it and it fixed my issues connecting to my carrier but it broke the sound. Then I saw your comment, "Old bootloader
(no KNOX :highfive
=> you need to flash exactly the following LTE modem and no other : XXUBMH1. Otherwise, sound / Wi-Fi may break.", and instead of concluding that I may need to upgrade the bootloader I concluded that I needed to downgrade the LTE modem to XXUFNB9.
Again, it was only ignorance on my part. I should have spent slightly more time studying this process.
Anyway, I found a thread with bootloaders and modems and settled on XXUHQC1. I decided Odin looked easier to use than Heimdall so I booted up Windows (which may have been the most painful part of the process). I flashed the modems and the bootloader. It didn't work. I tried again and it worked but my sound remained broken.
I then decided to try out your Resurrection Remix suggestion. For some reason this appears to have fixed my issues. I haven't explored it much yet (because I only just installed it) but I think it looks promising.
Thank you for your help and for your Resurrection Remix suggestion. Also, if you don't mind being thanked 4 years after the fact, thank you for your brief guide to flashing modems. :good: