:: To use this script, make a "_input" and "_output" folder within the same directory.
:: Then, put the audio files in ".wav"-format together with a ".png"-image in the "_input" folder.
:: The image needs to have the same name as the audio file.
:: The output will be a ".mp4"-video within the "_output" folder. The name will be the same as the audio file.
:: And yes, it loops through all the ".wav"-files within the "_input" folder.

@echo off
SETLOCAL ENABLEEXTENSIONS DISABLEDELAYEDEXPANSION
for %%f in (_input\*.wav) do (
  set "audio_file=_input\%%~nf.wav"
  set "cover_file=_input\%%~nf.png"
  set "output_file=_output\%%~nf.mp4"
  setlocal ENABLEDELAYEDEXPANSION
    ffmpeg -loop 1 -i "!cover_file!" -i "!audio_file!" -vf "scale=-2:720" -c:v libx264 -tune stillimage -c:a aac -strict experimental -b:a 320k -pix_fmt yuv420p -shortest "!output_file!"
  endlocal
)
ENDLOCAL