Reducing the bit rate will involve re-encoding, which means that you'll have to create separate output files. You could use avconv from the command-line:

avconv -i input.mp3 -c:a libmp3lame -b:a 128k output.mp3

To do a whole directory of .mp3s:

for f in ./*.mp3; do avconv -i "$f" -c:a libmp3lame -b:a 128k "${f%.*}-out.mp3"; done

This will create files with -out.mp3 at the end of their names. If you want to replace your originals, you can then use mv to overwrite them (warning: this should be considered irreversible):

for f in ./*.mp3; do avconv -i "$f" -c:a libmp3lame -b:a 128k "${f%.*}-out.mp3" && mv "${f%.*}-out.mp3" "$f"; done

It might be safer to do this in two steps:

for f in ./*.mp3; do avconv -i "$f" -c:a libmp3lame -b:a 128k "${f%.*}-out.mp3"; done

for f in ./*-out.mp3; do mv "$f" "${f%-out.mp3}.mp3"; done

You can do this to files recursively (every .mp3 in the working directory and all sub-directories):

shopt -s globstar

for f in ./**/*.mp3; do avconv -i "$f" -c:a libmp3lame -b:a 128k "${f%.*}-out.mp3"; done

for f in ./**/*-out.mp3; do mv "$f" "${f%-out.mp3}.mp3"; done