I think it must be from the Adriatic. A branch of that family arrived in France via Brooklyn and came from Pesaro where they still have family. But there are other parts of the family from Southern Italy, plus their Italian community comes from many places. However, they do not speak standard Tuscan since they have never lived in Italy or studied Italian.Jammo (with j=i and the final vowel reduced to a schwa) is Napoletan for "andiamo" (and the infinitive is "ghi" or "ji" or something of that sort). If the second syllable (mo) wasn't there, I don't know which daughter Sicilian.dialect of Latin it was. The Adriatic daughter languages of Latin (with the exception of Venetian), due to their somewhat peripheric situation, aren't as famous as Napoletan or
I'm going to look up this poemD'Annunzio's mastery of the Italian language was so great that I cannot even imagine he had to contrive something to make it fit. On the whole the language of his "Francesca da Rimini" is rich to the point of exuberance, very complex & highly stylised. Some would even say artificial or precious (in the literary sense of the word). As a consequence, the songs had to be all that to an even higher degree in order to be recognisable as poetic language within the already highly poetic "colloquial" language of this drama.
I think one aspect was that he was trying to imitate late Medieval or early Renaissance forms of poetry with its complex rhyme structures (both internal and verse-final), and the other was that the sensations triggered by the sonority of a single word were very important for his poetics (I suggest you read one of his most famous poems, "La pioggia nel pineto"). D'Annunzio treated rhythm very freely (see the same poem), so I don't think he did restrict himself in this way.
I love the word "tramezzino". Come to think of it double qq is rare. Normally q doubles as cq like in "acqua".By the way, the invention of the word "tramezzino" is attributed to D'Annunzio, as well as - according to my Italian language teacher at the university - the only Italian word with two q in a row: soqquadro.