I needed all my flac as mp3 to play them on a device that does not support .flac. Heres how to convert them all:

export IFS="                                                                                                                                          
"
for file in `find /home/claw/Musik -name *.flac` ; do echo "converting $file" && nice -19 sox $file `echo $file | sed 's/\.flac/\.mp3/g'`   ; done

You may have to install sox and libsox-fmt-mp3:

apt-get install sox libsox-fmt-mp3