Convert MP4 to MP3 - extract audio
Pull the audio out of any MP4 as an MP3 file. Wave decodes only the audio track and re-encodes it with LAME at 192 kbps - a rate that's audibly transparent for most source material.
What actually changes
Every MP4 file is really two tracks stacked together: the video track (the moving picture) and the audio track (the soundtrack). Extracting audio means keeping the audio track and throwing away the video track. The audio inside an MP4 is almost always in a format called AAC, which is the standard audio codec (the compression method) MP4 was designed around. MP3 is a different, older codec, so the extraction always includes a re-encode - Wave has to decompress the AAC audio, then re-compress it as MP3. Wave uses LAME (a well-tested open-source MP3 encoder used for two decades) at 192 kbps, which is the bitrate at which most listeners genuinely cannot tell the MP3 from the source on consumer headphones or speakers.
A quick note on "kbps" (kilobits per second): this is how much data the encoder spends per second of audio. Higher numbers = better quality and larger files. 192 kbps is the widely-accepted "sounds transparent on any consumer gear" threshold for music; voice-only content (podcasts, audiobooks, meetings) sounds clean at much lower rates like 96 or 128 kbps. Going above 192 kbps (256 or 320 kbps) makes the file larger but produces no audible improvement on any playback equipment most people own. Going below 128 kbps starts showing audible artefacts on complex sounds like cymbals, sibilants, and applause. Wave defaults to 192 kbps because it's the sweet spot - transparent for essentially everyone, at reasonable file size. Audio codecs explained covers where MP3 sits vs. AAC, Opus, FLAC, and the rest.
The extract itself is fast because it never touches the video. Wave discards the entire video track (which is the vast majority of the file's bytes) and only decodes the audio - a much lighter operation than decoding video frame by frame. An hour of MP4 audio typically becomes an ~85 MB MP3 in under 30 seconds on a modern laptop. Compare that to a full MP4 re-encode, which for the same hour of video would take several minutes.
Because MP3 plays on literally every device that plays sound - phones, cars, speakers, TVs, old MP3 players, embedded software, everything - it's the safest format to hand to someone whose playback environment you don't control. That's the whole reason this extract exists: to give you a version of the audio that just works everywhere, from the video file you already have. For the built-in ways to do it on a Mac, Windows or iPhone, see How to extract audio from a video; for why 192 kbps is the default, see MP3 bitrate explained.
Three steps.
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Step 1
Drop the MP4 onto the card.
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Step 2
On the inspect screen, pick MP3 as the output format.
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Step 3
Convert. Save the MP3.
Answers, specifically.
Will the MP3 sound as good as the source video's audio?
For any music content played on consumer speakers, headphones, or car stereos - yes, effectively identical. 192 kbps is the well-established point where trained listeners struggle to reliably tell an MP3 from the source in blind tests. Voice content (dialogue, podcasts, lectures) sounds completely transparent at this bitrate; it would still sound clean at half the bitrate. The tiny difference from the source only becomes audible on studio-monitor listening of tonally-dense music, and even there it takes practice to hear.
Can I choose a different MP3 bitrate?
Not yet. Wave ships a single 192 kbps default because it's the transparent-quality sweet spot for essentially any content on essentially any consumer playback. Bitrate options are on the roadmap. If you need a specific bitrate today (say, ultra-low 96 kbps for a slow connection, or maximum 320 kbps for a high-fidelity archive), a desktop tool like Audacity or the ffmpeg command line handles it - though for typical use, 192 kbps is what you want.
Why MP3 and not AAC or WAV?
MP3 is the only audio format that plays on literally every device that plays sound - old MP3 players, car stereos, embedded systems, phones, computers, everything. AAC is technically more efficient (smaller file at the same quality) and plays on all modern devices, but a small share of older hardware refuses it. WAV is uncompressed and roughly 8x larger than MP3 with no audible improvement for playback. Wave targets MP3 on purpose because it's the safest bet: give a stranger an MP3 file and it will play. See Audio codecs explained for the full audio-codec map.
What if the MP4 has no audio track?
Wave detects that during inspect and stops with a friendly message before any conversion runs.
Does the ID3 metadata carry over?
Basic tags (title, artist if present) are copied. Album art doesn't transfer through the current pipeline.
Can I extract just a portion of the audio?
Not yet. Wave extracts the full audio track; trimming lands in a later release. Until then, a desktop tool like Audacity or ffmpeg ('ffmpeg -ss 00:01:30 -to 00:03:00 -i input.mp4 -vn -c:a libmp3lame -q:a 2 output.mp3') can trim on the way out.
What if my MP4 has multiple audio tracks (director commentary, dubs)?
Wave extracts the default-flagged track. Multi-track extraction is on the roadmap. If you need a specific alternate track today, you can use ffmpeg with '-map 0:a:1' (or the appropriate index) to pick a specific stream.
How does 192 kbps MP3 compare to the AAC source?
For almost any content on any consumer playback gear, they sound identical. AAC is more efficient bit-for-bit at low bitrates, but at 192 kbps MP3 the difference is well below the audible threshold on speech, music, and podcasts. Only critical listening on studio monitors with tonally-dense material would ever separate them.
Is it legal to extract MP3 from an MP4?
Yes for the files you own or have permission to convert - the extract creates no copy of copyrighted content that did not already exist in the file you had. The audio track is already on your device inside the MP4; pulling it out into its own file is a technical operation on your own bytes. Ripping audio from a video someone else owns (a paid stream, a purchased download you did not buy) is a separate question about the source, not about the extract step.
How long does extracting audio take?
Roughly one-tenth of the video's runtime on a modern laptop. A 60-minute MP4 typically becomes a 90 MB MP3 in under a minute - Wave never has to decode a single frame of video, so the extract is bounded by audio-decode speed, not the video re-encode cost of a full transcode.