What's inside an MP4's soundtrack
An MP4 is a container, and the sound inside is almost always AAC, a compressed, already lossy format. Turning it into MP3 means decoding the AAC and compressing it again as MP3. That second round discards a little more detail, though at 192 kbps or higher it's very hard to hear.
If what you really want is the best possible copy of the sound, choose Original audio, no re-encode: the AAC track is saved as an .m4a file, bit for bit as it was in the video. Now and then an MP4 carries MP3 audio instead, and then the same option hands you an MP3 with no re-encoding at all.
MP3 still earns its place. It plays on older car stereos and MP3 players, opens in every editing app, and satisfies upload forms that only accept .mp3.
Choosing a bitrate, with file sizes
The bitrate decides both quality and size, and nothing else affects the size: a 30-minute talk at 128 kbps comes out at about 29 MB, whatever is in it.
- 128 kbps, speech: lectures, meetings, interviews, voice-overs. About 1 MB per minute.
- 192 kbps, the all-rounder for a mix of talk and music. About 1.4 MB per minute.
- 256 to 320 kbps, music you care about. 320 kbps is the highest bitrate MP3 supports, at 2.4 MB per minute.
A useful rule: MP3 needs a bit more data than AAC to sound the same, so choose a step above the source bitrate listed in Soundtrack in your file, 192 kbps for a 128 kbps AAC track, for example. Going far beyond that only makes the file bigger.
What people turn MP4s into MP3s for
- A recorded webinar, lecture or interview to listen to on the move, or to publish as a podcast episode.
- Your band's set, a recital or a rehearsal, saved as audio you can replay anywhere.
- A voice track for a transcription service, or skip that step and generate subtitles straight from the MP4.
- A short clip for a ringtone or an intro, cut to length with the MP3 cutter.
From there, tidy the MP3 in the audio editor, trims, fades, fewer long pauses, and lift a quiet recording to podcast loudness with Normalize Audio.
Small details, already handled
- The name carries over,
team-call.mp4becomesteam-call.mp3, so files stay easy to match up. - Mono stays mono, a single-microphone recording isn't padded out to fake stereo, so every bit goes into the one channel that matters.
- The sample rate is kept, usually 48 kHz for video, which every MP3 player handles.
- Silent videos are caught, if an MP4 has no audio track at all, you're told straight away instead of getting an empty file.
- Your MP4 isn't touched, the MP3 is a new file; the video stays exactly as it was.