convert audio format opus (native) to mp3

mkv2mp4

for file in *.mkv
do 
if  ! [  -f "${file%.*}.mp4" ] ; then 
echo "${file}" && ffmpeg -i "${file}" -acodec mp3 -vcodec copy "${file%.*}.mp4" 
fi
if ! [[ -z $(grep '[^[:space:]]' "${file%.*}.mp4")  ]] ; then
echo "Convert successful"
rm -f "$file"
else 
echo "Convert failed, keep mkv as it is"
rm -f "${file%.*}.mp4"
fi 
done