.m3u file extension is not a popular format like mp3 or mp4. It is quite special.
M3U is a text file saved with “.m3u” filename extension for emailing or for listening to Internet radio. M3U file may contain a multimedia playlist or URLs or some pathnames to the media files and/or folders. M3U file is not an actual file. It has a stream which is often used in downloading from a website. That is why the M3U files we have are quite small, most in Kbyte.
To convert M3U to MP3 or MP4, we cannot treat it like a common format and find any software to do it. There are quite little software can manage it even though some ads they can do it. We had better use VLC media player to stream .m3u files and then convert them freely.