I was under the impression you could just switch the file extension from "MP4" to "MP3". You should be able to do that with VLC. For multiple files you can write (read: copy, paste, edit, save) a batch file to do multiples for you. I know this is listed as solved, but I also wanted to recommend the Free Studio suite.
First of all, choose or make drag and drop of mp4 format video file that you want to convert into mp3 audio format. Step 2: Now, press “Convert”. Step 3: Finally, your converted mp3 audio track is ready to download. Just press “Download” and save mp4 as mp3 online.
tUF3xRQ.