How to convert a video to MP3
This is different from muting a video: you want the sound saved as its own file. Most devices have no built-in way to do that.
Converting versus muting
Muting a video, or removing its audio track, gets rid of the sound and keeps the silent video. Converting a video to MP3 is closer to the opposite: keep the sound, discard the video frames, and save the audio as a standalone file you can put on a music player or send separately. They are two different operations even though both involve separating audio from video.
On an iPhone or Android
Neither the Photos app nor the built-in camera roll has a way to export just the audio track of a video. Voice Memos and similar apps only record new audio; they don't extract it from an existing video file. This is one of the more common "how do I" gaps on mobile, since it is a fairly specific, less common task.
On a Mac with QuickTime
QuickTime Player can export audio only: open the video, choose File, Export As, and pick Audio Only. This saves an M4A file, not an MP3 directly, so a follow-up conversion step is needed if MP3 specifically is required (many devices and players accept M4A just as well, so check whether that's actually necessary first).
On Windows
There is no built-in equivalent in the Movies & TV or Photos apps for extracting audio from a video as a separate file. This is a task Windows genuinely has no native tool for, which is why most people search for a converter rather than a menu option.
A direct converter to MP3
Stayput's video to MP3 tool does this directly, on your phone or computer: drop in a video (or several, in a batch) and it extracts the audio and saves it as an MP3 or WAV, in the browser, without uploading the file. It skips the extra M4A-to-MP3 conversion step that QuickTime's export leaves you with.
Questions
Is converting a video to MP3 the same as muting the video?
No. Muting removes the audio and keeps the silent video; converting to MP3 keeps the audio as its own file and discards the video.
Why does QuickTime save an M4A instead of an MP3?
M4A is Apple's preferred audio format and what QuickTime's "Audio Only" export produces by default; it is not the same file extension as MP3 even though both are compressed audio, so some further conversion or a different tool is needed for MP3 specifically.
Does the audio quality change when converting to MP3?
MP3 is a compressed format, so there is some quality loss versus the original video's audio track, though it is usually not noticeable for speech or typical video soundtracks at a reasonable bitrate.
Can I convert just part of a video to MP3?
Trim the video to the part you want first (or trim the resulting audio file afterward), since most converters, including a browser-based one, convert the whole file by default.