What an M4A file actually is
M4A is the audio-only member of the MP4 family: the same kind of container as an MP4 video, minus the picture. Inside is usually AAC, the compressed format Apple uses for Voice Memos and iTunes Store music, and one that many Android recorder apps use too.
AAC is a capable codec, at the same bitrate it generally sounds better than MP3, so if everything you use plays M4A, there's no need to convert. People switch to MP3 for older car stereos and players, some Windows programs, course and website upload forms that only accept .mp3, and the occasional colleague whose software opens nothing else.
You'll also find M4A files exported from GarageBand, saved by dictation and call-recording apps, and among downloaded podcast episodes. If a file plays in your phone's music app but nowhere else, converting it to MP3 is usually the quickest fix.
Voice memos to MP3
To get memos off an iPhone, open Voice Memos, tap a recording and use the share button: Save to Files, or AirDrop to a Mac, gives you an .m4a you can drop here. The Voice Memos app on a Mac has the same share option.
Memos are speech, so 128 kbps is all they need, about 1 MB per minute. Recorded a long meeting or lecture? Tighten it first in the audio editor, which removes long pauses in one click, then turn the MP3 into text with Auto Subtitles.
Clearing out a backlog? Select a week's worth of memos and add them all at once. Each converts in turn and keeps its name with an .mp3 ending, and the finished set downloads as a single ZIP, no renaming afterwards.
Choosing a bitrate when the source is AAC
Converting AAC to MP3 is a re-encode, and MP3 is the less efficient of the two. To keep the sound you have, give the MP3 a little more room than the original had:
- Voice memos and calls, 128 kbps.
- Podcasts and interviews, 128 to 192 kbps.
- Music bought from the iTunes Store (256 kbps AAC), 256 or 320 kbps.
Going higher than that won't recover anything the AAC encoding removed; it only makes the file bigger. Mono memos stay mono, too, which is part of why 128 kbps sounds so clear for a single voice.
M4A files that won't convert
- Apple Lossless (ALAC): Voice Memos recorded with the Lossless audio-quality setting, and music imported as Apple Lossless, store ALAC inside the .m4a. Browsers have no ALAC decoder, so these files are flagged in the list instead of converted. Switching Voice Memos back to Compressed in the Settings app avoids this for future recordings.
- Copy-protected tracks, songs downloaded through a music-streaming subscription are encrypted, and no legitimate converter can open them.
Ordinary AAC files, including plain .aac ones, convert normally.