.... 
 echo "flac -c ... && rm -rf '$a'"
done | xargs -P 4 -n 1 -d $'\n' bash -c

#!/bin/bash
my_super_function()
{
    input="$1"
    OUTF=`echo "$a" | sed s/\.flac$/.mp3/g`
	ARTIST=`metaflac "$a" --show-tag=ARTIST | sed s/.*=//g`
	TITLE=`metaflac "$a" --show-tag=TITLE | sed s/.*=//g`
	ALBUM=`metaflac "$a" --show-tag=ALBUM | sed s/.*=//g`
	GENRE=`metaflac "$a" --show-tag=GENRE | sed s/.*=//g`
	TRACKNUMBER=`metaflac "$a" --show-tag=TRACKNUMBER | sed s/.*=//g`
	DATE=`metaflac "$a" --show-tag=DATE | sed s/.*=//g`

	flac -c -d "${a}" | lame -V 0 \
		-m j \
		--cbr \
		-b 192 \
		--noreplaygain \
		-q 0 \
		--lowpass 20.7 \
		--add-id3v2 \
		--pad-id3v2 \
		--ignore-tag-errors \
		--tt "$TITLE" \
		--tn "${TRACKNUMBER:-0}" \
		--ta "$ARTIST" \
		--tl "$ALBUM" \
		--ty "$DATE" \
		--tg "${GENRE:-12}" \
		- "$OUTF"

	rm -rf "${a}"
}
export -f my_super_function
find -type f -iname '*.flac' | parallel --no-notice my_super_function '{}'

my_super_function() {
        local l tag val
        local ARTIST TITLE ALBUM GENRE TRACKNUMBER DATE
        while read l; do
                tag=${l%%=*}
                val=${l#*=}
                case "${tag^^}" in
                 ARTIST) ARTIST=$val;;
                 TITLE) TITLE=$val;;
                 ALBUM) ALBUM=$val;;
                 GENRE) GENRE=$val;;
                 TRACKNUMBER) TRACKNUMBER=$val;;
                 DATE) DATE=$val;;
                esac
        done << EOF
$(metaflac --show-tag=ARTIST --show-tag=TITLE --show-tag=ALBUM --show-tag=GENRE --show-tag=TRACKNUMBER --show-tag=DATE "$1")
EOF
        flac -c -d "$1" | lame -V 0 -m j --cbr -b 192 --noreplaygain -q 0 \
                                 --lowpass 20.7 --add-id3v2 --pad-id3v2 \
                                 --ignore-tag-errors --tt "$TITLE" \
                                 --tn "${TRACKNUMBER:-0}" \
                                 --ta "$ARTIST" --tl "$ALBUM" \
                                 --ty "$DATE" --tg "${GENRE:-12}" \
                                 - "${1%flac}mp3"
        rm -f "$1"
}

PARALLEL=4
jobs_is_parallel_available() {
        local -i i
        local l
        while true; do
                i=0
                while read l; do
                        i+=1
                done < <(jobs -r)
                [[ $i -lt $PARALLEL ]] && break
                sleep 0.1s
        done
}

while read l; do
   ### ASYNC
   my_super_function "$l" &
   jobs_is_parallel_available
done < <(find / -type f -iname '*.flac')
wait