M4A
MP3's successor in an MP4 box.
Where it came from
M4A is not a codec but a container: the MP4 box holding AAC (or ALAC) audio, popularized by Apple's iTunes in the mid-2000s. The ".m4a" extension was Apple's way to mark "audio only, no video."
AAC inside delivers better quality than MP3 at the same bitrate, which is why M4A became the default for purchased and ripped music on Apple devices.
How it works
Structurally M4A is an ISO Base Media file (like MP4) whose tracks are audio atoms (moov, mdat). The codec is usually AAC, sometimes ALAC for lossless.
UWC converts M4A ↔ WAV/MP3 and more in the browser studio, decoding the AAC stream natively when the browser allows.