Use an online converter
Upload the MP4 file
Select MP3 as the output format
Start the conversion
Download the MP3 file
Use VLC Media Player
Open VLC
Go to Media
Select Convert/Save
Add the MP4 file
Choose Convert
Select MP3 as the profile
Start the conversion
Save the MP3 file
Use FFmpeg
Install FFmpeg
Open a terminal or command prompt
Run ffmpeg -i input.mp4 output.mp3
Check the output MP3 file
Use Audacity
Install Audacity
Import the MP4 file
Export as MP3
Save the MP3 file