What is inside an MP4 file
MP4 is a container that holds several streams. A typical MP4 from a phone, a screen recorder or Zoom has one video stream, usually H.264 or HEVC, and one audio stream, almost always AAC. The two are stored side by side with timing information so a player can keep them in sync.
Converting MP4 to MP3 means opening the container, decoding the AAC audio stream and encoding that audio again as MP3. The video stream is simply left behind. Nothing from the picture ends up in the MP3, and the original MP4 on your device is not changed.
Because the video is usually the bulk of the file, the MP3 is far smaller. A one-hour phone video is often more than a gigabyte, while the audio from the same hour at 192 kbps is roughly 85 MB.
MP4 files people usually convert to MP3
Phone videos. iPhones and Android phones record MP4 or MOV. When the useful part is what someone said, such as a talk filmed from the audience or a family story told at dinner, an MP3 is easier to keep and send than the video.
Screen recordings. Built-in screen recorders and most capture tools save MP4. If you recorded a tutorial, a demo or an online class, the narration is often all you need afterwards.
Lectures and online courses. Recorded lectures are usually distributed as MP4. Pulling the audio out lets you listen on a walk or in the car, and turns a video library into something that fits on a phone.
Zoom and other meeting recordings. Zoom saves a local recording as an MP4 plus a separate M4A file with only the audio. If you still have that M4A, you may not need to convert the MP4 at all. If only the MP4 was kept, converting it gives you the same audio as an MP3.
M4V files from Apple devices are the same container with a different extension and convert the same way.
Choosing the MP3 bitrate: 128, 192 or 320 kbps
The converter writes constant bitrate MP3 with the LAME encoder. Three settings are offered.
- 128 kbps: enough for speech. Lectures, meetings, interviews and narration sound the same to most listeners at this setting, and the file is the smallest of the three, about 1 MB per minute.
- 192 kbps, the default: a safe choice when you are not sure what is in the file. Speech is well covered and most music sounds fine. Around 1.4 MB per minute.
- 320 kbps: the highest MP3 bitrate. Choose it for music, live performances and anything you plan to edit further. Around 2.4 MB per minute.
A higher bitrate cannot add quality back
The audio inside an MP4 is already AAC, which is lossy, and MP3 is lossy too. Each conversion discards a little more detail. A higher bitrate reduces how much is lost in the MP3 step, but it cannot restore anything the AAC encoder already removed. For a phone video or a Zoom recording, 128 or 192 kbps gives a result you will not be able to tell apart from the original.
Converting also cannot fix the recording. Wind noise, echo in a lecture hall and a speaker far from the microphone stay exactly as they were in the video.
The conversion runs in your browser
This converter does not upload your video. It uses FFmpeg compiled to WebAssembly, which lets the same conversion engine used on servers run inside a web page. When you pick a file, the browser reads it from your disk, converts it and holds the MP3 in memory until you press Download. Nothing is sent to RocketScribe and nothing is stored on our side.
The first time you use it, the browser downloads the engine, about 32 MB, and caches it. Later conversions start straight away, and the page keeps working offline. It handles one file at a time. The practical limit is about 2 GB per file on a desktop; phones and tablets manage less because browsers give pages less memory there.
The MP3 keeps the original file name with an .mp3 extension. It exists only in the open tab, so download it before you reload or close the page. Current Chrome, Edge, Firefox and Safari 16 or newer are supported.
If you actually need the words
Many MP4 to MP3 conversions are a first step toward a transcript. After the conversion, a Transcribe this file button sends the MP3 into the transcription box lower on this page. That part runs on RocketScribe's servers, so the audio is uploaded for it, unlike the conversion. You get a timestamped transcript you can edit in the browser, in 90+ languages, and export as TXT, DOCX, PDF, SRT, VTT or CSV. You can try it without an account.