Compress all FLAC (or MP3) files in subfolders to OPUS

Compress all FLAC audio files in subfolders to OPUS at 128 Kbit/s (near-transparent), while maintaining the metadata tags and album art.

find ./ -name '*.flac' -exec bash -c 'ffmpeg -i "$0" -f flac - | opusenc - --bitrate 128 --vbr --comp 10  "${0/flac/opus}" && rm -v "$0"' {} \;

We use find to select all the flac files, and run the bash files. The first runs ffmpeg and pipes the output to opusenc. The reason is if you use ffmpeg to save to opus files, the album art is lost, whereas when piped in to opusenc, it is encoded properly.