Convert M4A to MP3
Turn an Apple recording into the format everything else plays.
How it works
M4A is a filename convention rather than a codec. The file is an MP4 container carrying one audio track and no video, and Apple adopted the extension so that people would stop double-clicking .mp4 files and waiting for a picture. Two quite different codecs arrive wearing it.
Most of the time the track is AAC: iPhone Voice Memos, iTunes and Apple Music purchases, anything recorded by a Mac. Occasionally it is Apple Lossless, which compresses without discarding anything and produces files three to five times larger. This page reads both and does not need to be told which it has.
Why anything still wants MP3
AAC is the better codec. It was standardised as MPEG-4 Audio in 1997, uses a longer transform window and finer frequency resolution than MP3, and at 128 kbps it beats MP3 at the same rate by a clear margin in listening tests. None of that helps when the device in front of you refuses to open the file.
The refusals are specific and stubborn: car head units built before roughly 2012, older Sony and SanDisk players, gym equipment with a USB socket, cheap voice recorders, some conference and church sound systems, and a long tail of Windows software that was finished before AAC became common. All of them read MP3, because everything reads MP3.
What the transcode costs, and what it drops
Going from AAC to MP3 decodes and re-encodes, so a second round of lossy compression is applied on top of the first. Encoding at a higher bitrate than the source gives the MP3 encoder enough room that the difference stays below what most people can hear. An Apple Lossless source has no such problem, since the decoded signal is exactly what was recorded.
Apple's own metadata is where the losses that people notice actually happen. Track, artist, album and year are carried across. Cover artwork, grouping, sort fields, ratings, play counts and any purchased-item information are not, because they live in Apple-specific boxes with no MP3 equivalent. Chapter markers in an audiobook M4A are also dropped, which matters if the file is a long recording you navigate rather than a song you play.
Protected files from the iTunes Store before 2009 carry FairPlay encryption and cannot be read at all. Everything sold since is unprotected and converts normally.
Questions
Does this work on iPhone Voice Memos?
Yes, and it is the most common use of this page. A memo recorded on a recent iPhone is AAC in an M4A container, usually mono at 32 kbps for the standard setting or considerably higher for lossless mode. Speech at 64 kbps mono comes out clear and small. The recording's date and title survive if the phone wrote them.
Is my file uploaded?
No. The container is parsed and the AAC track decoded by your own browser, then re-encoded by a WebAssembly build of LAME running in a Web Worker. Open the Network tab and convert: the encoder module downloads once and nothing else is requested. A voice memo can be a private thing, and this page is built so that privacy is a property of the architecture rather than a promise.
What if my M4A is Apple Lossless rather than AAC?
It converts the same way and the result is slightly better, because the decode gives back the original signal rather than an already-compressed approximation of it. The file you start with will be much larger, three to five times an AAC of the same recording, and the MP3 you finish with will be the same size either way.
Which bitrate should I use for a spoken recording?
64 kbps mono is enough for an interview, a lecture or a memo, and turns an hour into roughly 29 MB. 96 kbps mono if the recording has background music or several people at different distances from the microphone. Reserve 192 kbps and above for music, where the second encode has more to preserve.