Wow! Two years have passed, and I didn't expect a reply to this one
Anyway, I was able to get the Norwegian characters visible in mutt by setting the LC_CTYPE variable in my shell. You can try typing "export LC_CTYPE=no_NO" just before executing mutt. If this works, you can add it to your shell's rc-file (probably .bashrc) to make it permanent.
Note that you need to have the "no_NO"-locale installed for this to work.