I dislike MP3 but less than I dislike WMA :-) Anyway, my car CD player wouldn't want to play WMA. So:
1. Install Medibuntu repository in 1 line
sudo wget http://www.medibuntu.org/sources.list.d/$(lsb_release -cs).listoutput-document=/etc/apt/sources.list.d/medibuntu.list && sudo apt-get -q update && sudo apt-getyes -q --allow-unauthenticated install medibuntu-keyring && sudo apt-get -q update
2. Install FFMpeg
sudo apt-get install ffmpeg libavcodec-extra-52
3. Launch FFMpeg conversion
for i in *.wma; do ffmpeg -i "$i" -ab 160k "${i%wma}mp3"; done