Unfortunately, you cannot convert a playlist file (in this case .m3u file) to an actual audio file (for example .mp4), which means m3u to mp4 conversion is not possible. Playlists just contain references to the location of the audio files, not the audio data itself. At best you can open the playlist in a text editor like Notepad and check the path to the audio files where you can find them.
Open M3U file Open MP4 fileA music library management program - renamed to Helium (version 12) and replaced in 2016