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The beauty of the public domain, of course, is that there are no *rules* for sourcing, and you don't *need* to do any at all! But you're right that it's still considered good etiquette to source the work you use if it doesn't interfere with your own creativity to do so, it's just not strictly necessary. It's also a good idea for the purposes of the jam, so we can easily see that the entry does indeed make use of at least one 1927 work.

The creator name and work title is generally all you need - and you can include more information if you wish. You can include them in the actual rulebook or somewhere else in the game materials, or just put them in the game's description on its Itch page if you prefer!

We're excited to see what you come up with! Digging deeper is always a great idea - and of course there's a whole prize category (Deep Cut) just for that. In the past we've seen people find some obscure works and turn them into incredibly creative games - like this puzzle/narrative game based on a pair of science journal articles, or this deeply moving Holocaust retrospective based on a lesser-known painting.

No Discord at the moment, but that's a good idea - maybe we'll set one up. If so we'll let everyone know! Though it might be something we just plan for next year's jam.

As for the rules: the game must in some way involve a work that entered the public domain this year. That doesn't necessarily mean you need to directly use the work itself, but it still needs to play a role in the game somehow. Either way, you can still incorporate lots of your own original art/music/etc. Maybe you're retelling a story from a 1926 novel or film with your own twist, or mashing several together, but all the actual material (art, music, etc.) is your own! Or, conversely, maybe you're using lots of art from 1926 as graphics to make a completely new game that has nothing to do with the original context of the art - maybe even the 1926 works are just background imagery. Or somewhere in between! As long as it incorporates at least one newly-public-domain work, somehow. Note that this doesn't mean the game must be set in 1926 - though that's certainly an option.

The composition is definitely in the public domain, including in the US. Different recordings of it, however, may still be under copyright.  So, yes, you should look to see if you can find a recording that is confirmed as being in the public domain.  I know that, last year, once the composition went into the public domain, a few people recorded new versions to be used in the public domain, so you might be able to find a recording that has been clearly set up for such use.

Wow! That looks amazing. And nice use of a 1924 public domain song! I do hope you'll continue to make the game even if it's too late for the jam. That is really impressive.