use of comm and sort tools

[Linux for blind general discussion]

I wrote previously about ffmpeg and audio variable bitrate. After conversion, there are some files that did not convert. I would like to compare two listings and discover which ones are missing. So, we have these commands:
find . -type f -name \*.m4a | sed -e 's at .*/@@' -e 's/\.4a$//' > m4a.txt
find raw2 -type f -name \*.mp3 | sed -e 's at .*/@@' -e 's/\.mp3$//' > mp3.txt
Now I want to run comm and have it dump to another file which lines in m4a.txt do not exist in mp3.txt. How would I go about doing that? Or is there a better way?