Convert M4A to MP3

An .m4a can be two completely different things

M4A is a container, and the same extension covers a lossy file and a lossless one. Most .m4a files hold AAC — an iTunes purchase, a voice memo, audio pulled from a video — and converting one to MP3 is a second generation of lossy compression, best handled by encoding a little above the source bitrate so the new pass adds nothing audible. But an .m4a can equally hold ALAC, Apple's lossless codec, which is a perfect copy of the original recording. Converting that to MP3 is a single clean pass from a flawless source, like converting a FLAC, and the file is worth keeping afterwards because every future conversion should start from it rather than from the MP3.

You can usually tell them apart by size. A three-minute ALAC file runs to twenty or thirty megabytes; the same song as AAC is three or four. If you are about to delete the original, that number is worth a glance.

Audiobooks lose their chapters, and this is the most common disappointment here. An .m4b — the audiobook form of the same container — carries chapter markers that let a player jump between sections and remember where you were. MP3 has no equivalent in a single file, so a converted book becomes one long unmarked track. Artwork does not come across either. For a book, converting is usually the wrong move altogether.

Copy-protected purchases cannot be opened by this or by anything else. A .m4p from the old iTunes Store, or a downloaded Apple Music track, is encrypted to an account rather than merely encoded, and there is no key available to us and none we would be entitled to use. The page says so plainly rather than failing vaguely. MP3's own limits apply as always: two channels at most, 48 kHz at most.

When you want something else

For an audiobook, keep the .m4b. Every current phone, and every audiobook app worth using, plays it with its chapters intact — which is the entire reason the format exists. If a specific old player forces the issue, ffmpeg -i book.m4b -f segment -segment_times ... can split it into one MP3 per chapter, which at least keeps the structure as filenames.

For music, ffmpeg keeps the tags and artwork this drops: ffmpeg -i in.m4a -c:a libmp3lame -q:a 2 -map_metadata 0 out.mp3. And before converting at all, it is worth checking what actually needs it — AAC in an .m4a plays on essentially every device made this century, and the file you have is already better than the one you are about to make.