Extract MP3 audio from MP4 video
Pull the soundtrack out of a video file. Why M4A is usually the better target, and how to avoid stacking lossy compression twice.
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What you are doing here is extraction rather than conversion in the usual sense: an MP4 file is a container holding separate video and audio streams, and you want to keep the audio and discard the picture.
The double-compression problem
The audio inside an MP4 is almost always AAC, which is already lossy. Converting it to MP3 means decoding that lossy stream and re-encoding it with a different lossy codec. Each generation discards detail, and the second pass cannot recover what the first one threw away — it can only add its own losses on top.
You will mostly not hear this on laptop speakers or earbuds. It becomes audible on decent headphones in cymbals, sibilance, and reverb tails, where lossy codecs struggle most.
Consider M4A instead
If audio quality matters, target M4A rather than MP3. M4A can hold the original AAC stream, so the audio is copied out of the container untouched — no second encode, no additional loss, and usually a smaller file than the equivalent MP3. It plays natively on iOS, macOS, Android, Windows, and every current browser.
Choose MP3 when you specifically need it: older car stereos, gym equipment, DJ software with fussy import rules, cheap MP3 players, and some podcast platforms still expect it. Compatibility is the reason to pick MP3 — quality is not.
Bitrate guidance
Since you are already one generation down, do not compound the loss by encoding too low. For music, 192 kbps is a sensible floor and 256 kbps is comfortable. For speech — lectures, interviews, podcasts — 128 kbps is plenty and 96 kbps is often acceptable for mono voice.
Encoding above the source bitrate gains you nothing. A 128 kbps AAC track re-encoded at 320 kbps MP3 is a large file containing the same degraded audio.
Multiple audio tracks
Some MP4s carry several audio streams — alternate languages, a commentary track, or a stereo mix alongside surround. Extraction picks the first or default track. If you need a specific one, that requires stream selection beyond what a simple conversion offers.
Before you extract someone else's video
Ripping audio from material you do not own is copyright infringement in most jurisdictions, regardless of how easy the tooling makes it. Extracting audio from your own recordings, licensed material, or content released under a permissive licence is fine. Downloading a music video to keep the song is not.