I have more music than would fit onto the memory card of my phone. This is largely due to the 256 kBit/s versions that are sold now. On the phone, I do not really hear the difference between 256 kBit/s and 128 kBit/s, but I can clearly see the difference in size.

This script creates a whole copy of your music library, just with a smaller bitrate. It uses either ffmpeg or lame to convert the tracks.

Just use:

make
sudo make install

mp3-packer path-to-music

For more information, see mp3-packer -h or the manpage