OK, used gedit to change source.list to this:
#
deb
http://ftp.us.debian.org/debian/ stable main contrib non-free
deb
http://ftp.us.debian.org/debian/ stable-updates main contrib non-free
deb
http://security.debian.org/ stable/updates main
deb
http://ftp.debian.org/debian stretch-backports main
with a hard return after that last line. Then saved it, exited terminal, and attempted to install sound converter. I then got this message:
Enable Third-Party Software?
Sound Converter is not free and open software, and is provided by "(null)". This software source must be enabled to continue installation. Enable and Install? -> <Enter>
Error Message: installing not available.
I did not reboot in between sessions, but did not think it was necessary.
After that, I went back into the software update gui and looked up the repositories listed there. All of the previous options were no longer checkmarked. Nothing showed up. I then rechecked all of them and tried again. I got the same original error then. I went back to gedit and this is what showed up:
deb
http://ftp.us.debian.org/debian/ stable main contrib non-free
deb
http://ftp.us.debian.org/debian/ stable-updates main contrib non-free
deb
http://security.debian.org/ stable/updates main
deb
http://ftp.debian.org/debian stretch-backports main
deb
http://deb.debian.org/debian/ stretch main contrib non-free
deb
http://security.debian.org/ stretch/updates contrib main non-free
deb
http://deb.debian.org/debian/ stretch-updates contrib main non-free
I'm at a loss as to what to try next.