find . -name "*.flac" -type f -print0 | while read -d $'\0' f; do ffmpeg -i "$f" -c:a libfdk_aac -vbr 3 "${f%.flac}.m4a"; done

find . -name "*.flac" -type f -exec sh -c 'ffmpeg -i "$f" -c:a libfdk_aac -vbr 3 "${f%.flac}.m4a"' _ {} \;

なるほど
https://askubuntu.com/questions/35922/how-do-i-change-extension-of-multiple-files-recursively-from-the-command-line

s/f/1/
変数間違えた

このregexpも間違ってるけどまぁいいや