narration_post_processing.sh

function join(){
# join *.mp3 narration files in current dir

    MOVIE_FILE="join.list"
    
    if [ -f $MOVIE_FILE ]; then
        rm $MIVIE_FILE 
    fi
    
    target_list=""
    for i in `ls 0*.mp3`
    do
        echo "file $i" >> $MOVIE_FILE
    done
    
    ffmpeg -f concat -safe 0 -i $MOVIE_FILE -c copy joined.mp3
    rm $MOVIE_FILE
}

function normalise (){
# normalise sound level

    ffmpeg-normalize $1 -c:a libmp3lame -o normalised.mp3
}

function filter (){
# noise canceler

    ffmpeg -i $1 -af "lowpass=3000" filtered.mp3
}

join
normalise joined.mp3
filter normalised.mp3

rev.2 to rev.3

afftdn=nr=10:nf=-40:tn=1 is removed. If both this option and lowpass filter were applied, noise reduction did not work properly on my SM58.