#!/usr/bin/env bash
# shellcheck disable=SC1004
# shellcheck disable=SC2016
# shellcheck disable=SC2126
set -eux
test $# -gt 0 ||
set -- "${0%/*}"/{old,new}/audio
#preserveCoverArt="-vcodec copy -f mkv"
find "$@" -depth -type f \( -name "*.mp3" -o -name "*.m4a" -o -name "*.audio*" -o -name "*.unknown_video" \) -print0 | sort -n -z |
xargs -0 -P "$(lscpu --online -p | grep -v "#" | wc -l)" -I {} bash -c '
  nice -n 19 ffmpeg -y -i "$0" \
    -map_metadata 0:s:a:0 \
    -c:a libopus -b:a 32k \
    -application voip "${0%.*}.opus" &&
  rm -vf "$0"
' {} \;

# -map_metadata 0:s:a:0
# copies metadata from the first input file (0:),
# first audio stream (s:a:0) to all output files.