#!/usr/bin/env bash
# shellcheck disable=SC1004
# shellcheck disable=SC2016
# shellcheck disable=SC2126
set -eux
test $# -gt 0 ||
set -- "${0%/*}"/{old,new}/audio
find "$@" -depth -type f -name "*.mp3" -print0 | sort -n -z |
xargs -0 -P "$(lscpu --online -p | grep -v "#" | wc -l)" -I {} bash -c '
  nice -n 19 ffmpeg -y -i "$0" \
    -c:a libopus -b:a 64k \
    -application voip "${0%.*}.opus" &&
  rm -vf "$0"
' {} \;