I was reading a description of the musical "light in the piazza" and it described the music as having "extended melodic structures" Does anyone know what's that means?
This may qualify as more of a guess than an answer, but it is based on experience with musical theatre.
In the classical period of the Broadway musical, dominated by Jerome Kern, Rogers & Hart, Rogers & Hammerstein, Irving Berlin and many other wonderful composers, the music was almost all song form, with discernable verses, choruses, refrains and other traditional song elements, full of repetitions, echoes and tightly related musical phraseology.
Stephen Sondheim began his illustrious composing career in the heart of this tradition, providing lyrics for Westside Story as well as complete scores for other important pieces toward the end of the era.
However, with the scores of such shows as A Little Night Music, Sweeny Todd and others, his songs increasingly took on an extended form reminiscent of the recitative in opera.
Andrew Lloyd Weber and his contemporaries took this development further, with an ultimate expression in the musical version of Les Miserables, whose composers I have forgotten.
So a critic making such an observation might be remarking on a score's conformance with the current musical theatre vogue of avoiding traditional song form and supplying both chorus and principal players with music that is freer in form, shying away from the repetitions and tropes of the traditional songs that characterized Broadway's golden age.
Let me know if that makes sense in the context of your question.
I'm going to say the term has no significant meining and I'll show you why by picking out each word.
The traditional meaning of extended is as follows:
made larger; enlarged. - "an extended kitchen and new balcony"
lasting longer than is usual or expected; prolonged. - "an extended period of time"
Another possibility when using extended with music is the idea of Extended Harmony which is pretty much utilizing notes above an octave (9ths, 11ths, 13ths).
The meaning of melodic is as follows:
of, having, or producing melody. - "melodic and rhythmic patterns"
pleasant-sounding; melodious. - "his voice was deep and melodic"
I've also hear the term melodic used to describe all vocal parts including background vocals which are not melodies.
When you put these three terms together especially without much context you could be talking about longer then usual melody lines, use of extended harmony in the melody, having background vocals create extend chords, or any combination of the definitions above or none of them as they may misunderstand one of the terms.