flac is a popular one among a lot of audiophiles - idk if there's more niche stuff out there (there probably is, haha).
I have only dug around in image metadata stuff, not audio, but in terms of resources, ExifTool has a truly insane level of documentation on metadata formats and file formats, and it looks like it's got some audio and video formats on top of about a million image formats.
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I have only dug around in image metadata stuff, not audio, but in terms of resources, ExifTool has a truly insane level of documentation on metadata formats and file formats, and it looks like it's got some audio and video formats on top of about a million image formats.