Use GNU parallel and fmpeg command for convert flac audio files
Install need packages
apt install -y parallel ffmpeg
Convert to OPUS
parallel ffmpeg -y -i {} -map_metadata 0 -id3v2_version 3 {.}.opus ::: **/*.flac
Convert to OGG
parallel ffmpeg -y -i {} -map_metadata 0 -id3v2_version 3 {.}.ogg ::: **/*.flac
Conver to MP3
parallel ffmpeg -y -i {} -map_metadata 0 -id3v2_version 3 {.}.mp3 ::: **/*.flac