How to convert MP4 to MP3 on Windows — nothing to install, nothing uploaded

What is inside an MP4

.mp4 is a container — one file holding separate tracks. A typical video carries a picture track and a sound track side by side, and the player reads both at once. Getting an MP3 out of it therefore means two things: dropping the video track, and re-encoding the audio as MP3, because the sound inside an MP4 is almost never MP3 to begin with.

This is what FFmpeg reported for the 55.7-second clip used in every test below — a vertical video with narration, made earlier for a blog post:

Notice how much of the file is picture. The video track is the bigger share, which is why an audio-only version comes out several times smaller before the MP3 encoder has even chosen a bitrate.

Method 1 — PowerShell, with what Windows already has

Windows ships Media Foundation, the same media pipeline its own apps use, and PowerShell can call it through WinRT. Paste the block below into a PowerShell window, change the folder path on the line marked with a comment, and every MP4 in that folder gets an MP3 beside it. The videos themselves are left alone, and nothing is sent anywhere.

Add-Type -AssemblyName System.Runtime.WindowsRuntime
$ms = [System.WindowsRuntimeSystemExtensions].GetMethods()
$op = ($ms | Where-Object { $_.Name -eq 'AsTask' -and $_.GetParameters().Count -eq 1 -and $_.GetParameters()[0].ParameterType.Name -like 'IAsyncOperation*' })[0]
$ap = ($ms | Where-Object { $_.Name -eq 'AsTask' -and $_.GetParameters().Count -eq 1 -and $_.GetParameters()[0].ParameterType.Name -like 'IAsyncActionWithProgress*' })[0]
function Wait-Op($o, $t) { $x = $op.MakeGenericMethod($t).Invoke($null, @($o)); $x.Wait(-1) | Out-Null; $x.Result }
function Wait-Act($a) { $x = $ap.MakeGenericMethod([double]).Invoke($null, @($a)); $x.Wait(-1) | Out-Null }

[Windows.Storage.StorageFile,   Windows.Storage, ContentType=WindowsRuntime] | Out-Null
[Windows.Storage.StorageFolder, Windows.Storage, ContentType=WindowsRuntime] | Out-Null
[Windows.Media.MediaProperties.MediaEncodingProfile, Windows.Media, ContentType=WindowsRuntime] | Out-Null
[Windows.Media.Transcoding.MediaTranscoder, Windows.Media, ContentType=WindowsRuntime] | Out-Null

$folderPath = "C:\Users\you\Videos"         # folder holding the .mp4 files
$folder  = Wait-Op ([Windows.Storage.StorageFolder]::GetFolderFromPathAsync($folderPath)) ([Windows.Storage.StorageFolder])
$profile = [Windows.Media.MediaProperties.MediaEncodingProfile]::CreateMp3("High")

Get-ChildItem "$folderPath\*.mp4" | ForEach-Object {
    $src  = Wait-Op ([Windows.Storage.StorageFile]::GetFileFromPathAsync($_.FullName)) ([Windows.Storage.StorageFile])
    $dst  = Wait-Op ($folder.CreateFileAsync([IO.Path]::ChangeExtension($_.Name, ".mp3"), 1)) ([Windows.Storage.StorageFile])
    $t    = New-Object Windows.Media.Transcoding.MediaTranscoder
    $prep = Wait-Op ($t.PrepareFileTranscodeAsync($src, $dst, $profile)) ([Windows.Media.Transcoding.PrepareTranscodeResult])
    if ($prep.CanTranscode) { Wait-Act ($prep.TranscodeAsync()); "$($_.Name) -> $($dst.Name)" }
    else { "$($_.Name): $($prep.FailureReason)" }
}

Each converted file prints one line, such as lecture.mp4 -> lecture.mp3. A file that cannot be converted prints its reason instead, and the loop moves on to the next one, so one bad file does not stop the folder.

Which quality preset — measured

The block uses the High preset. Replace "High" with "Medium" or "Low" to trade quality for size. All three, from the same 55.7-second video:

Encoding took under half a second for nearly a minute of sound; starting PowerShell takes longer than that. For speech — a lecture, a meeting, a voice note — Medium is usually plenty and keeps files small. For music you intend to keep, use High. The MP3 cannot sound better than the AAC it was made from, so there is nothing to gain by going higher than the source.

Method 2 — FFmpeg, if you already have it

FFmpeg is not part of Windows, but plenty of tools bundle it, and it is worth knowing the two commands because each covers a case PowerShell does not. These results come from FFmpeg 7.1 on the same clip.

To MP3, smaller at high quality

ffmpeg -i input.mp4 -vn -c:a libmp3lame -q:a 2 output.mp3

-vn drops the video track and -q:a 2 asks the LAME encoder for high-quality variable bitrate. The result was 918,769 bytes, averaging 131 kb/s. Variable bitrate spends fewer bits on quiet stretches, which is why it came out smaller than Windows' fixed 192 kbps while aiming at a similar quality.

Keep the original audio — no re-encoding

ffmpeg -i input.mp4 -vn -c:a copy output.m4a

-c:a copy lifts the AAC track out exactly as it is. Nothing is decoded or compressed again, so nothing more is lost: 2,221,936 bytes at the original 317 kb/s. If the place you are sending it accepts M4A — most phones, browsers and messaging apps play it — this is the cleanest result there is. It just is not MP3, so use it only when MP3 is not required.

Why renaming to .mp3 does not work

It is the first thing many people try, so I tested it. I copied the MP4 and renamed the copy to .mp3. Windows labelled it audio/mpeg from the name alone — then reported its length as 0:00 and its bitrate as 0, because the contents are not MP3. The file was still 5,369,904 bytes, the video was still inside, and its first bytes still spelled ftyp, the signature every MP4 starts with.

An upload form that checks only the extension may let it through. Anything that actually decodes MP3 — a car stereo, an older player, a stricter upload check — will not. The extension is a label; the bytes decide the format.

When it fails

A video with no sound. I stripped the audio track from the test clip and gave it to the same code. CanTranscode came back False with the reason Unknown — no mention of the missing track. Screen recordings made without audio capture are the usual way to end up here. Play the video and check for sound before suspecting the script.

Reading the result too early. If you add your own steps after the conversion — moving the MP3, uploading it — keep the waits in place. A file checked before the transcode finishes looks truncated or empty.

Other file types. The block only picks up files matching *.mp4. I did not test other video formats here; change the pattern if you need them, and watch the printed reasons.

The options side by side

Online converters work, but the entire video leaves your PC — including whatever was said in a recorded meeting or a family clip. For anything private, the first two rows do the same job without the file going anywhere.

Only need part of it? Trim after converting

A one-hour lecture as a 192 kbps MP3 is over 80 MB; the ten minutes you actually need is under 15 MB. Convert first, then drop the MP3 into the audio cutter on this site and keep just the section you want. It runs in your browser, so the recording is not uploaded either.

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