ffmpeg -i LPCM_audio_test.mp4 -c:v copy -c:a pcm_s16le LPCM_audio_test-le.mkv
Kiel, I'm a bit rusty with batch files, but if you need to fix a lot of MP4s and do it from within Windows Explorer, I suggest you create a batch file, place it on your desktop and drag-and-drop 1 or more MP4s on to this icon.
for %%F in (%1) do (
echo ffmpeg -i %%F -c:v copy -c:a pcm_s16le %%~nF.mkv
if errorlevel 1 goto ERROR_OCCURRED
if not "%1"=="" goto FILES_LOOP
echo An error occured when trying to process %filename%
At the moment the "echo" in "echo ffmpeg -i %%F -c:v copy -c:a pcm_s16le %%~nF.mkv" will just show you the command line that would be executed, but not actually run FFMPEG. So if you drop 3 MP4s on to the CONVERT.BAT icon you should see 3 command lines. To activate it, remove this particular ECHO and re-save the batch file.
BTW, if FFMPEG is not in your path list, you can hardwire its full path into the batch file. To get the full path of a file in Windows Explorer, rather than just r.clicking on FFMPEG, instead shift-r.click on it. This will show an extra option: "Copy as path".