MP4 to MP3

Pull the audio track out of an MP4 video and save it as an MP3. The video is decoded only to be thrown away, and the audio is re-encoded to MP3 rather than copied.

MP4 to MP3 keeps only the sound from an MP4 clip, encoding it fresh as MP3 while every video frame gets dropped. Quality picks one of three presets, medium by default, through a LAME encoder mediabunny loads on demand. Encoding runs on WebCodecs, so a browser tab is needed to open this page.

Questions

Does this keep the video?

No. Only the audio track is decoded and kept; the video is discarded entirely rather than converted or attached to the MP3.

What quality is lost?

MP3 is lossy, so the audio is re-encoded at the chosen quality, low, medium or high, medium by default, rather than copied. This is one generation of lossy encoding on top of whatever audio codec the MP4 already used.

Which engine handles this, and where can it run?

mediabunny and WebCodecs decode the audio, and since no browser encodes MP3 through WebCodecs itself, mediabunny loads a LAME-based MP3 extension on demand as a separate chunk to do the encoding. This is browser only; the command line and MCP refuse it and point to the web page.

What size should I expect?

A real MP3 well over a kilobyte for a couple of seconds of audio in the fixture test, comfortably past what an empty or corrupt file would produce. The exact size tracks the quality option chosen.

What is the reverse page?

There is none. MP3 to MP4 is not one of the named pairs, since MP4 is not one of the output formats Convert audio offers.

Related Video tools

n→nM4A to MP3Convert an M4A (AAC in an MP4 audio container) to MP3. Both are lossy, so this is a second lossy encode on top of the first: some quality that AAC already discarded stays gone, and MP3 discards more of its own.n→nWAV to MP3Compress an uncompressed WAV recording into MP3. This is the one pair here where real compression happens for the first time, so the file shrinks by roughly a factor of ten at the default quality.n→nMP3 to WAVDecode an MP3 into an uncompressed WAV. Nothing MP3 already discarded comes back; the result is only a format older software and hardware samplers can read, several times larger for the same audio.n→nOgg to MP3Convert an Ogg Vorbis file to MP3 for a player or device that never learned Vorbis. Both codecs are lossy, so this decodes Vorbis and re-encodes to MP3 rather than repacking the same compressed bytes.n→nFLAC to MP3Compress a lossless FLAC down to MP3. This is the one direction here where real audio quality is thrown away on purpose, traded for a file roughly a tenth the size at the default quality setting.n→nMP3 to OggRe-encode an MP3 as Ogg Vorbis for a game engine or open-source pipeline that expects it. Both codecs are lossy, so this decodes the MP3 and encodes fresh Vorbis frames rather than repacking anything.n→nAAC to MP3Convert a raw ADTS AAC stream to MP3 for hardware and older apps that never added AAC decoding. AAC is usually the higher-quality codec at a given bitrate, so this trades some of that quality for wider compatibility.n→nOpus to MP3Convert an Opus file, the codec behind most voice notes and Discord recordings, to MP3 for software that cannot open it. Opus stays small at low bitrates that make MP3 audibly worse, so the MP3 usually ends up the larger file.