Put the following in a batch file. You will need to download ffmpeg and place it in some path.
https://ffmpeg.org/download.html


--------------- Start --------------
@echo off
set ffmpeg="C:\Tools\Utils\ffmpeg-win-2.2.2\ffmpeg.exe"
set infolder=C:\Temp\In-WAV
set outfolder=C:\Temp\Out-MP3
set infiletype=wav
set outfiletype=mp3

echo Starting!
echo Input folder: %infolder%
echo Output folder: %outfolder%
echo Input filetype: %infiletype%
echo Output filetype: %outfiletype%
echo.

cd /d "%infolder%"
if not exist "%outfolder%\." md "%outfolder%"
dir
echo Starting!
for %%f in (%infolder%\*.%infiletype%) do (
if exist "%outfolder%\%%~nf.%outfiletype%" echo Skipping '%%~nf.%infiletype%' since '%outfolder%\%%~nf.%outfiletype%' exists.

if not exist "%outfolder%\%%~nf.%outfiletype%" (
echo.
echo.
echo -------------------- Starting '%%f'
echo Converting '%%f' to '%outfolder%\%%~nf.%outfiletype%'
%ffmpeg% -i "%%f" "%outfolder%\%%~nf.%outfiletype%"
)
)

echo.
echo.
echo All Finished!
echo.
timeout 10
----------------- End -----------------