>When one looks at the description on the back of a symphonic CD,
>one is likely to find each movement characterized by the tempos to
>be employed-allegro, moderato, presto, etc. Yet in an opera, sections
>tend to be designated by their form-aria, cavatina, duetto, etc. I've
>concluded that this is because most orchestral movements are composed
>of some version of sonata form. I have noticed that sometimes other
>forms are identified (and, in these cases the tempo is omitted) as with
>menuetto and trio, rondo, theme and variations, etc.
>Am I on the right track? Is this a convention with a rationale or is
>it simply tradition?
Well, it is not necessarily a matter of sonata form in orchestral movements
because they are not all in sonata form, as you note yourself. Sometimes
composers depart from the conventional sequence of tempos even in four
movement symphonies, and sometimes movements flow into one another without
pause (in the score and on some programs/CD notes it will say "attacca,"
as in Schumann's 4th Symphony, where all of the movements are so connected).
The tempo or form indications in the program gives the listener--especially
the first time listener--an idea of what to expect in such cases--and
maybe a basis for judgment on the performance: "You call that an adagio!?"
In the score the tempo indications are going to be much more detailed--and
varied--than what is listed as the main tempo of a movement, though some
CDs do give you a lengthy breakdown of some movements.
With operas, since they are long, the CD tracks may be listed by the
forms you mention both to give you an idea of where you are, and to
let you select a particular favorite number, I assume.