Where M4A recordings come from
M4A is the MPEG-4 container with audio only, almost always holding AAC. Apple devices use it by default, which is why most M4A files people need to convert are voice memos. The Voice Memos app on iPhone, iPad and Mac saves every recording as M4A.
Android is a mixed picture. Many Android voice recorder apps also produce M4A, while some write 3GP, a related container that this converter reads as well.
Zoom writes two files for a local recording: an MP4 with the video and an M4A with just the audio. The M4A is the one to convert if you only need the sound. Browser recordings made in Safari, for example with web-based recorders, are often M4A too.
Why some devices and apps refuse M4A
MP3 is older than AAC and became the default audio format before M4A was common. Almost anything that plays sound supports it. M4A support is wide on phones and computers but patchy elsewhere.
Typical cases: a car stereo that reads a USB stick but only lists MP3 files, an older portable player, a phone system or voicemail greeting that wants MP3, and apps or upload forms that accept MP3 but reject M4A. An editing tool may want MP3 for the same reason.
Converting M4A to MP3 fixes all of these with one file. The output keeps the original name with an .mp3 extension, so you can tell at a glance which recording it was.
What changes when AAC becomes MP3
Both AAC and MP3 are lossy. The AAC encoder in your phone already removed detail it judged inaudible, and the MP3 encoder does the same again on what is left. The second loss is small, and for a voice memo recorded on a phone microphone it is not something you will hear. Music loses slightly more, which is why a higher bitrate is worth choosing for it.
Converting cannot improve the recording. Room echo, distance from the microphone and background noise stay exactly as they were. Keep the original M4A if you might need it later; the MP3 is a copy made for compatibility.
Which bitrate to choose
The converter writes constant bitrate MP3 using the LAME encoder, with three choices. A bitrate higher than the source does not add quality: if the memo was recorded at a low AAC bitrate, choosing 320 kbps for it only makes a bigger file with the same sound. For anything that is mostly voice, 128 kbps is the sensible pick, and 192 kbps if you want a margin.
- 128 kbps for voice memos, meetings, interviews and dictation. Speech is fully intelligible at this setting and files stay around 1 MB per minute, so a 30-minute memo is about 30 MB.
- 192 kbps, the default, when the recording mixes voice and music or you simply want a safe middle setting.
- 320 kbps for music and for recordings you may edit again later. About 2.4 MB per minute.
Nothing is uploaded
The conversion runs inside your browser using FFmpeg compiled to WebAssembly. Your M4A is read from your device, converted there and never sent to RocketScribe. We store nothing, because nothing reaches us. That is the right model for voice memos, which are often personal notes, medical or legal conversations, or recordings of other people.
On first use the browser downloads the engine, about 32 MB, and caches it; afterwards the page also works offline. One file converts at a time. The practical limit is about 2 GB per file on a desktop, far larger than any voice memo; phones manage less. The MP3 exists only in the open tab until you press Download, so save it before closing the page. Current Chrome, Edge, Firefox and Safari 16 or newer are supported, so the conversion works on the iPhone that made the memo.
If you actually need the words
A lot of M4A to MP3 searches are really about getting a voice memo transcribed. You do not need the MP3 for that, since RocketScribe transcribes M4A directly. After a conversion, the Transcribe this file button sends the MP3 into the transcription box below. That step runs on our servers, so the audio is uploaded for it. It returns a timestamped transcript in 90+ languages that you can edit and export as TXT, DOCX, PDF, SRT, VTT or CSV, free to try without an account.