It feels like the desire here is for dialogue as close to song as possible without being sung. Not merely spoken lyrics, but a song like quality of some nature. From your apt description. If that makes sense. Which would be fittingBut those are all elves addressing mortals. Where we know elves adjust their speech down to the mortal level. And if we have elf on elf dialog - we have them through Bilbo's lense when he wrote the Silmarilion - he was not there - as such he delivers the content of those speeches as they were reported to him after ages (and we are told elves do not remember those verbatim, but only the meaning that was expressed - otherwise their language would not change much over time - or at least the oldest elves would remember all the previous stages/versions of their own language - which they explicitly do not - they still need scholars for that) or from songs or legends - not the literal original speech.
Clear, not slowed down (as in the discussed scene elves speak among themselves, and not to mortals), unrestrained, competent, poetic, melodious, precise with meaning, pauses and intonation - with a feel of mastery over language as a concept and its actual physical verbal expression, with actual joy in the performance - savouring the words. - No mumbling of any kind.