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Convertessa uses FFmpeg to take the audio stream out of each MP4, decode it and re-encode it as MP3, discarding the video entirely. MP3 is a lossy format, so you set the quality that fits the material before the run starts. Drop one file or a folder of recordings and the whole queue is processed on your Mac — no upload, no account.
It is tempting to picture this as lifting the audio out of the video intact, and for an M4A that is roughly what happens. MP3 is a different story. The audio inside an MP4 is almost always AAC, and AAC data cannot be poured into an MP3 file — the two codecs are unrelated. FFmpeg decodes the AAC to raw samples and re-encodes them with LAME, so what lands on disk is a second-generation lossy file.
That is not a disaster, but it is worth knowing. Each encoder throws away what its psychoacoustic model says you will not hear, the two models disagree, and the artefacts the AAC encoder left behind become raw material for the MP3 encoder. A clean source at a generous quality setting is indistinguishable in ordinary listening; an already-thin source gets thinner.
One quirk confuses almost everyone. MP3 encodes in frames of 1,152 samples and LAME needs a priming period, so it records its encoder delay and padding in a header at the front of the file. Players that read that header reproduce the original length exactly; players that ignore it leave a few milliseconds of silence at the start, which is why gapless albums sometimes tick between tracks in older software. The compensation is size: strip the video from an hour-long recording and hundreds of megabytes become tens.
Drag the MP4s onto Convertessa, or point it at a folder of lecture captures, interviews or screen recordings. Every file is queued at once and processed one after another, locally.
Choose MP3, then set the quality for the material: speech and lectures stay perfectly clear well below what music needs, and the difference in file size across a folder is large.
Click Convert. FFmpeg decodes the AAC and hands it to LAME, writing one MP3 per input. Durations can differ by a few milliseconds because MP3 stores audio in fixed-size frames.
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Slightly, in principle, because two lossy encoders are in the chain: the AAC inside the MP4 already discarded information, and LAME discards more on top of it. With a decent source and a generous quality setting, the difference is not something you will pick out on speakers, headphones or in a car. With a source that was already heavily compressed — a low-quality download, a video call recording — the second pass has less to work with and it becomes audible.
MP3 carries one or two channels, so a 5.1 or 7.1 track has to be downmixed. The centre channel, where nearly all dialogue lives, is folded into both stereo channels along with front left and right, and the surrounds arrive at a lower level. Speech usually ends up quieter relative to music and effects than it was in the original mix. Nothing is broken — the spatial information simply has nowhere to go.
The default track, which is the first audio stream the file declares. An MP3 holds a single audio stream, so there is no way to keep both in one output. To capture the other language, convert into a container that keeps multiple tracks and take the audio from there. Check the result before deleting the source: a mislabelled default track is common on discs ripped to MP4, and the language you get is not always the one you expected.
Basic tags such as title, artist and album map from the MP4's metadata atoms into ID3 frames. Chapters are the real casualty: in an MP4 they are a track in the file, and the ID3 equivalent is supported so patchily that podcast and audiobook apps often ignore it. Embedded artwork is not something to count on either. If chapters matter for a long lecture or an audiobook, M4A and M4B keep them properly where MP3 does not.
MP3 encodes in frames of 1,152 samples, and LAME needs a priming period, so it records its encoder delay and padding in a header at the front of the file. Players that read the header trim it and reproduce the original length; players that ignore it leave those few milliseconds in place. It is a property of MP3 itself rather than of this conversion, and it is the reason gapless albums can tick between tracks in older software.
Point the CLI at the folder and let the queue work through it:
convertessa ./Lectures/*.mp4 --to mp3 --quality 80 -o ~/Audio
There is no bitrate field — quality runs on a 0–100 scale, where higher keeps more data and produces a larger file. For speech, a middling setting is ample, and the size drop is dramatic: an hour of screen recording that occupied several hundred megabytes as video typically lands in the tens of megabytes as MP3.
If everything that has to play the file supports it, yes. The audio in an MP4 is already AAC, and an M4A container holds AAC natively, so no second encoding generation is involved and no resampling or downmixing is forced. MP3 wins on exactly one thing, and it is usually the deciding one: near-universal playback, including car head units, gym equipment, hardware players and software that has not been updated in fifteen years.
There is nothing to encode, so there is nothing to write. Screen recordings made without a microphone or system audio, silent b-roll from a camera, and animation exports converted to MP4 all land in this category. If you expected sound, check the source in QuickTime Player: a greyed-out volume control means the file has no audio stream, so nothing was lost in conversion — the problem is upstream, in whatever recorded it.