audio/mp4

An MP4 container with sound but no picture — the .m4a files that iTunes made ubiquitous. The container tells you nothing about the codec inside: it is usually AAC but can be ALAC or anything else the ISO base media format can hold, which is what the codecs parameter is for (codecs="mp4a.40.2" for AAC-LC). Apple's legacy audio/x-m4a still circulates and some servers emit it, but audio/mp4 is the registered name. The signature is the ftyp box at offset 4; the brand bytes that follow (M4A, mp42) vary by producer.

.m4a is audio, .m4b is the audiobook variant with bookmarking — same container, different convention.

Defined by RFC 4337 · IANA registration. Registry facts from the IANA media-types registry via mime-db.