If you do not want to convert
.wav files to MP3 format by hand and you have only Windows prompt on your hands, together with a
ffmpeg.exe (look here), the command to run is
for %f in (*.wav) do ( ffmpeg -i "%f" -q:a 2 "%~nf.mp3" )
-q:a 2 stands for variable bitrate; it is a standard FFMPEG parameter. You may use other FFMPEG parameters as well.
Look here for further Windows command-line improvements.