> So my first question would be, if someone records the output of that version, how does licensing work on that recording? Their browser is generating the audio.

> The current main version plays the same sounds, but pre-rendered, using cross-faded loops like you're describing. I considered the audio files to be purely an optimization, so I didn't consider licensing on those.

IANAL and my comment depends on US copyright law. I would argue that the source code files and the audio that the user's computer generates from the source code have the same encoding-decoding relation that an MP3 file does to the audio that the user's computer generates from the MP3. The copyright holder of the MP3 file has copyright control of the decoded audio even if the user's computer does the decoding, so you should have copyright control of the audio directly generated by your code on the user's computer. If you put a certain license on the source code but don't explicitly specify a license for the audio files you generated for Spotify and Apple Music, then I think the permissions granted by the license will apply to the audio generated by the user's computer but not necessarily to the audio generated by your computer. You could explicitly grant permit certain actions to the source code and explicitly prohibit those actions to the audio files generated by the user's computer, with the caveat that fair use might make some of the prohibitions on the latter unenforceable due to the permissions on the former. Since you are the author of the source code files, you can make the generated audio files you put on Spotify have an identical or different license from the repository files. You can specify both the license for the repository files and the license for the generated audio files you put on Spotify anywhere, including in the repository readme.

> So if the code is MIT-licensed and generates the audio, what's the license on the audio? The repository doesn't contain audio files, but code that generates them. Can I add a license for those non-existing files in the repository? Unfortunately, all I have is more questions.

If you put the code under the MIT license then the license on the audio generated from the code by the user's computer is the user's choice, because the MIT license is a permissive open-source and free license. If you put the source code files under the MIT license then the user can do just about anything with the source code files and anything derived from them as long as the user follows the few requirements in the license (in the case of the MIT license, providing correct attribution, preserving the license notice, and minding the lack of a warranty). (Well, the user also has to obey trademarks and patents on the original files. The MIT license doesn't have a patent grant, while the Apache 2.0 license frees the user from the author's patents on the files.) "just about anything" includes releasing copies or derivatives for any purpose under any license (including proprietary ones) of the user's choosing. I think of it as: the user can wrap the user's copy of MIT-license code into any license that doesn't prohibit inclusion of MIT-license code, but cannot remove or change the license of the original code.