Where audio metadata sits
An MP3 keeps its tags in a block at the front (ID3v2) and often a second one in the last 128 bytes of the file (ID3v1), with older APEv2, Lyrics3 and TAG+ blocks sometimes stacked behind it. A WAV is a RIFF file, so each kind of metadata is a chunk beside the audio: INFO fields, an ID3 tag, an XMP packet, the labels on cue markers. An M4A is an MP4 whose tracks are sound, and its tags are iTunes-style atoms inside the movie header.
In every one of them the metadata sits beside the samples rather than inside them, which is why a clean is a rewrite of the container and never a re-encode. The decoded audio of a cleaned file is identical to the original, sample for sample, and that is checked with ffmpeg outside this project rather than asserted here.
A song can contain a photograph that knows where it was taken
Cover art is an image, and images carry metadata of their own. One real MP3 in the test corpus holds artwork whose IPTC block names the photographer, their agency, their job title, and the city and country the shot was taken in — inside a music file, where nobody thinks to look for it.
So the artwork is opened and read as the file it is, and what it contains is listed in the report before you decide. Removing the tag removes the picture with it. An M4A can hold more than one image in a single artwork atom; each is reported separately, because a reader that stops at the first leaves the rest unmentioned.
What a field recorder writes about the person holding it
The Broadcast Wave extension — the bext chunk, from EBU Tech 3285 — has an originator field, and on professional equipment that field is routinely a person, a company, or a machine serial number, stored with the date and the time of day the recording was made. A session file in the corpus carries the recorder software, the date and the time to the second.
It is treated with the weight GPS gets in a photograph: named in the warnings, and removed by a clean. So is iXML, the production block that holds project, scene, take, notes and often the names of the crew. Both are removable rather than editable: they are fixed-layout production records, and the useful operation on somebody else’s name in one is removal.
What is deliberately left alone
The first frame of a variable-bitrate MP3 usually carries a Xing, Info or VBRI header, and inside it a LAME tag naming the encoder and its settings. It is metadata by nature, and it is left in place: removing it breaks seeking, and damaging a recording to hide the name of a program is not a trade this tool makes on your behalf. It is reported instead, encoder name and all, so the choice is visible rather than silent.
Cue marker positions stay too. The words somebody typed against a marker are metadata and go; where the marker sits is part of the shape of the recording and stays.