To extract the audio from an MP4, open it in the converter. This guide explains what the converter does with it and why.
Check the audio track, not the video codec
MP4 is a media container that can hold separately encoded video and audio tracks. H.264, HEVC or AV1 describes the video; AAC, MP3 or another audio codec describes the sound. Those are independent choices. PrivateMP3 lists the tracks and sends only the selected audio track through the extraction engine. It does not need to decode the video frames to make an audio file.
AAC is common in MP4, but the extension alone does not guarantee it. After the file opens, the Audio track control shows the codec, reported channel count and language when available. If the MP4 has several audio tracks, the control starts on the first one. Choose the intended track before making cuts or volume changes, because changing it later clears those edits after a confirmation.
Measured: an HEVC MP4 behaves like an H.264 one
Tested on 6 October 2026 in Chrome 154.0.8037.98 and Edge 154.0.4258.53 on Windows 11 (Intel Core i5-13600KF, 32 GB), with files generated by FFmpeg from a 220 Hz tone and quiet pink noise. Times are from Chrome; Edge differed by at most 0.7 s on any row. Each saved file was read back with ffprobe. The method is described in the screen recording test results.
The test file was five minutes of 1280x720 HEVC picture with 48 kHz stereo AAC at 128 kbps, 72,300,537 bytes. File check showed MP4, 1 audio track, 1 video track, AAC 128 kbps, and the editor was ready in 0.7 s.
| Output | Time | Saved size |
|---|---|---|
| MP3, 192 kbps | 3.0 s | 7,200,744 bytes |
| M4A, unchanged copy | 0.2 s | 4,869,993 bytes |
| WAV | 2.4 s | 57,600,044 bytes |
Those rates match the H.264 recordings in the test report: the picture is never decoded, so its codec changes neither the time nor the output. What matters is the audio track the panel lists.
Three output buttons, three different jobs
| Output | Processing |
|---|---|
| MP3 | Decode the selected track; encode a new 192 kbps MP3. |
| Unedited AAC → M4A | Copy the AAC packets without re-encoding. |
| Edited or non-AAC → M4A | Decode the track; encode new 192 kbps AAC. |
| WAV | Decode the track; write 16-bit PCM. |
PrivateMP3 has no bitrate selector. The MP3 and newly encoded M4A outputs use 192 kbps. The size estimate changes with the duration left after cuts, not with a quality setting that PrivateMP3 does not offer.
When M4A preserves the AAC you already have
AAC and MP3 are both lossy codecs. Turning AAC into MP3 requires decoding the AAC and making a second lossy encoding. If the destination accepts M4A, an unchanged AAC track can take a different route: PrivateMP3 copies its encoded packets out of the MP4 and places them in a new M4A container. The converter identifies this with the copy note, and the audio is not encoded again.
A cut or volume change prevents packet copy because the timeline or decoded samples must change. M4A then becomes a newly encoded AAC file. Merely choosing M4A is therefore not a promise of no quality loss; look for Copied without re-encoding — same audio, no quality loss.
The picture can fail to preview while audio still works
The Play control asks the browser's built-in media element to open the original MP4. The waveform and conversion use the selected audio track through a separate path. If Play is unavailable but the waveform reaches Ready, the audio track has still passed the checks needed for conversion. Conversely, seeing the picture or hearing native playback does not prove that the selected track can be decoded by the extraction path.
The saved MP3, M4A or WAV contains the selected audio only. Video and subtitle tracks are not copied into an audio output. Cuts are made against the audio timeline shown in the waveform.
Match the symptom to the next check
- No Audio track picker: if the editor opened, PrivateMP3 found one audio track and selected it.
- This file has no audio track: the MP4 opened, but no listed track was classified as audio.
- no decoder: that selected audio codec cannot be decoded in the current browser. A different track in the same MP4 may work.
- Convert stays disabled: wait for Ready. An eligible unchanged AAC-to-M4A copy is the decoder-free exception.
- Duration or waveform error: use the stage-by-stage diagnostic guide; renaming the file does not change its container or codec.