Last but not least, for Windows 10 users, the VLC media player is more than a free and user-friendly video player. Also, read how to automatically add subtitles to iMovie. Of course, if you only need parts of the audio in the MP4 video, you need to add the MP4 to the timeline and do some trimming and finally only save the MP3 audio of the MP4 video to a desirable place on your Mac using iMovie.
Click Next > rename your file name and location for saving the MP3 audio > click Save.