She talks quite a bit about Virginia Woolf's theory that rhythm of prose is primary, coming before word choice or sentence construction. It’s a silent rhythm of unvoiced stress, because we don’t usually read prose out loud. Hence the 'Wave in the Mind'. Interesting to read this in the week that Derrida died, as he writes a lot about the relation between spoken and written, in a kind of self-indulgent way.
Le Guin analyses the stress patterns of different writers, and suggests that the closer prose is to poetry, the greater the proportion of stressed syllables. My guess is that this pattern would be found in all European-derived languages. Conversely dry-as-dust academic and technical texts use less stress (a lot of connecting words, a lot of composite nouns, for example).
Also, if I'm interpreting her correctly, the closer prose is to folk formats, the more repetitive the rhythm. Thus she says the Lord of the Rings uses heavily stressed language, and a certain degree of rythmic repetition. Jane Austin uses stress more moderately, and has a longer, looser rhythm, and so on.