If we'd like to update the game after it's finished to include something like voice acting, are we allowed to do so as long as it continues to follow the jam's rules? The game itself would otherwise be a complete product and we are willing to add a note that this change was made.
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Right, thank you for reminding us about updating projects after the initial deadline.
In short: yes, that's allowed.
Any future updates we'd recommend making it available as a separate download, but this is not necessary. Generally we'd like to encourage everyone to keep their "Version 1.0" builds up for posterity's sake, as a way to show "this is what we accomplished in a week."