@JimmyHartington's solution would have worked too, except /sox was retained when he copy/pasted from your post. For completeness (although export isn't needed at the start of the first line):
export PATH=/opt/homebrew/bin:$PATH;
sox -m "$KMVAR_Local__mp3A" "$KMVAR_Local__mp3B" "$KMVAR_Local__OutputMP3"
This is a good method if you are making multiple calls to sox in the same "Execute Shell Script" action, because it saves you typing out the full path every time 
If you are going to use more "Execute Shell Script" actions, have a look at the wiki. The bit that was tripping you up is explained in the Path in Shell Scripts section, and you'll see that since your sox utility isn't located in a directory under any of the default paths you have to either add the location to $PATH (@JimmyHartington's method) or use the full path when you call it (mine).