Just as an additional thought for those of us who prefer simple solutions in a bash shell:
Decoding FLAC files is done with the ‘flac’ utility, and encoding MP3 is the job of ‘lame’. So, all you have to do is to pipe the output of ‘flac’ to ‘lame’, that’s all:
flac -d -o - inputfile.flac | lame - output.mp3
Something like
for flacfile in *.flac; do mp3file=${flacfile%.flac}.mp3; (flac -d -o - $flacfile | lame - $mp3file); done;
should do the same for all FLAC files in the current directory. (untested)