Improvised rap shows the plausibility of alternative universe examples.
Early freestyle rap was written down, but since 1990, it has focused on improvisation, or "coming off the top of the head". This mirrors "flyting", the Scottish tradition of rap battles described in Cracked.com's "6 Forms of Modern Depravity Way Older Than Your Grandpa". If people in real life can learn to improvise verse, it wouldn't be impossible for a culture to value the ability to improvise verse. Musicals and blank verse plays take place in such a culture. As for the tunes, a lot of Emily Dickinson poems can be set to the same songs because they're all written in "ballad meter", or iambic heptameter. Perhaps there are a few tunes that everyone knows and can employ in an improvised Filk Song.
The skills needed in such a culture are similar to those used in church meetings.
Consider the singing of contemporary hymns in Christian churches that put lyrics on a projector instead of placing song books behind seats (as churches using traditional hymns do) or giving people song books to take home (as Jehovah's Witnesses do).
These contemporary hymns are fairly repetitive, where the melody of two or four measures repeats twice to teach the melody and chord progression to the audience. The rhythm can be guessed from the lyrics, and the melody can be guessed from the rhythm and chord progression, in much the same way as jazz improvisation. The chorus and especially the middle eight are even more repetitive to let the audience join in.
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