For AI generated content it should maybe be:

["ai-generated", "true"]

Do this, because the first Music client that was implemented does this, and Nostria does the same now. Please don't do this differently as there is already many tracks like this.

OR... since the syntax on other items is "track_number", "sample_rate", it does not follow that "ai-generated" should be with dash and not underscore. So I guess we need to change this to "ai_generated" as suggested.

Doing "license name or URL" is not optimal. This requires a lot of logic on clients and there is no way to organize this properly.

The most commonly used licenses should be included in the NIP, so that apps use the same identifiers. Some clients might want to show different Creative Commons icons for the licenses and if this is different across clients, it will be trouble.

This is what Nostria has today:

All Rights Reserved, CC0 1.0, CC-BY 4.0, CC BY-SA 4.0, CC BY-ND 4.0, CC BY-NC 4.0, CC BY-NC-SA 4.0, CC BY-NC-ND 4.0

So license should instead of name and URL in same spot, it should be:

["license", "License Name", "URL"]

Maybe I also suggest adding some license regarding lyrics, how lyrics with timestamps should be formatted?

Do we really want to support both plain text and markdown for .content? That's a big difference in logic being applied to the content. I vote for plain text only, with a basic structure like already exists:

"Lyrics:
TEXT

Credits:
TEXT"

But with some additional details on how to do timestamps in the lyrics.

Thanks!