Why convert M4A to MP3
M4A is common on Apple devices but some car stereos, older players, editing apps and websites only accept MP3. Converting voice memos and music to MP3 makes them play everywhere.
M4A is M4A, AAC audio in an MP4 container, the format of iPhone voice memos and iTunes purchases; MP3 is MP3, the audio format every player, car stereo and phone plays.
What changes
- The sound is decoded and encoded again as MP3. At 192 kbps or more you won't hear a difference from the M4A.
- Sample rate and channels are kept (mono stays mono, stereo stays stereo).
On your device
Your files are converted by your browser and never uploaded, so there is no sign-up, no file-size cap beyond your free disk space and no daily limit. You can convert up to five files at once.
How this works
The file is read in slices by Mediabunny in a Web Worker, which finds the audio track; your browser's built-in decoders (WebCodecs) turn it into sound samples, which are then encoded as MP3 by LAME, the open-source MP3 encoder, compiled to WebAssembly and loaded only when needed, at the bitrate you choose. The result is written to the browser's private storage as it is produced, so long recordings don't need to fit in memory, then offered for download. Nothing is uploaded.
Frequently asked
Which bitrate should I choose?
192 kbps sounds like the original for most listening; 320 kbps for music you care about; 128 kbps for speech and the smallest files.
Are my files uploaded?
No. The conversion runs in your browser on this device; nothing is sent to a server.
Is there a size limit?
Only your device's free disk space. Long recordings are written to the browser's private storage as they are converted.
Sources
- ISO/IEC 14496-14 — MP4 file format
- ISO/IEC 11172-3 — MPEG-1 Audio (including Layer III, MP3)
- Mediabunny documentation — Converting media files
- LAME — open-source MP3 encoder
Reviewed Oct 19, 2026 · test vectors fixtures/convert/audio/m4a-to-mp3/