I recorded lots of tutorial videos for my students using a microphone which is not very nice, so here is an example to batch increase all video files’ volume for 6 db, and export to mp4 files.

FOLDER=$1  
for i in $FOLDER/*.mov;    
do 
    name=`echo $i | cut -d'.' -f1`;    
    echo $name;    
    ffmpeg -y -i "$i" -strict -2 -vcodec copy -af "volume=6dB" "${name}.mp4"; 
done