Sometimes arias are called scenas, aria or scena in the singular.
An aria.
Aria
Reduced paid up
That word could be aria.
There is no such thing as an "extended sentence" in grammar. "Extended sentence" is a legal term, not a grammatical term.
A cavatina is a musical term meaning a short simple song that does not have multiple verses or repetitions. A cavatina can be instrumental or vocal and is generally operatic.
That is the proper spelling of the operatic term "aria".
Relations.
Yes.
The term is 'aria-cabaletta'. An example being Violetta's aria from act 1 of La traviata - 'È strano! è strano……Ah! fors e lui' [aria] - 'Sempre libera' [cabaletta].
Apnea
Oh, dude, that's easy. You call it a "solo piece" or a "solo composition." It's like when your friend decides to go solo in a boy band - they just want all the spotlight to themselves, you know? So yeah, it's basically a piece of music written for one instrument to show off its skills without any distractions.