You can extract audio from MP4 and save it as MP3 using 7minti in your browser without sending the video to a server.
Extracting audio from MP4 is what most people mean when they search for MP4 to MP3. 7minti demuxes the audio track, decodes it, and writes MP3—skipping video data you do not need in the final file.
Questions about extract audio from mp4
Does extracting audio reduce quality?
MP3 is lossy, but starting from the embedded audio track at a high bitrate keeps results close for most listening.