Create Audio hierarchy, add Icarus

[ajhalme]

• Create Audio hierarchy
• Add 'Icarus' by Scott Buckley as a placeholder main menu theme --> Towards moddable audio OpenCiv3#984

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Prior or Modified Work

• I confirm that the assets contain no material derived from original game files or otherwise unlicensed copyrighted work except in cases of fair use.
• I have done my due diligence to confirm that any assets not original to me are freely redistributable under open-source compatible terms.
• For each separately sourced asset or set of assets, I have included or updated credits in each affected directory's README.md file containing the asset name or filename(s), source (URL if possible), any known license or attribution information, and if any modifications were made.

Original Work

• I hereby grant free use of original adapted assets I have created under CC-BY-NC-SA-4.0.
• Any new assets have been included in at least the highest-quality usable format as described in the wiki to the extent possible.
• (Recommended) I have included the raw working files used to produce my assets, if any, preferably in the formats recommended in the wiki.

General

• I have included only assets (or their corresponding metadata and working files) that are usable now in OpenCiv3, or will be usable with a current OpenCiv3 PR. No assets for future use only.
• I have checked the wiki for recommendations before submitting new placeholder assets.
• Any assets which are generated by AI, or otherwise intended as placeholders, have been listed as such in placeholders.yml.
• Any updated assets which are now considered production-ready have been removed from placeholders.yml.

[WildWeazel]

love it!

Only nitpick, since we're still figuring this out as we go, do you want to cite the license for your adaptation in the credits? I'm just thinking since there's an explicit license on the original, it's not obvious that your version falls under the "CC-BY-NA-SA-4.0 unless stated otherwise" umbrella.

[ajhalme]

My change is so minor, I'm not sure I can even claim copyright for an adaptation -- as such, probably easiest to just licence the "adaptation" with the licence as the original. CC-BY 4.0 has that "No downstream restrictions" bit, which potentially applies here.

[ajhalme]

Not super cool to force-push on someone else's branch, BTW

[WildWeazel]

L&SGTM

sorry to jump the gun on the rebase button- force of habit from work where I am the one who clicks