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" )

The parameter -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.