for f in *.flac; do flac -cd "$f" | lame -b 320 - "${f%.*}".mp3; done

or

for a in ./*.flac; do ffmpeg -i "$a" -qscale:a 0 "${a[@]/%flac/mp3}"; done

find . -type f -name "*.flac" -print0 | while IFS= read -r -d '' file; do echo "$file"; flac -cd "$file" | lame -b 320 - "${file%.*}".mp3; done