Just put mp3 files under the mp3 folder and it works.

#info kbps 
find . -maxdepth 1 -type f -iname '*.mp3' -print0 | while IFS= read -r -d '' f; do br=$(ffprobe -v error -show_entries format=bit_rate -of default=nw=1:nk=1 "$f"); if [ -z "$br" ] || [ "$br" = "N/A" ]; then printf "%s\tN/A\n" "${f#./}"; else kb=$(( (br + 500) / 1000 )); printf "%s\t%d kbps\n" "${f#./}" "$kb"; fi; done

#same volume all mp3
find . -type f -iname '*.mp3' -print0 | xargs -0 mp3gain -r -k

