Often, when passages have parentheses they're meant to convey an extra meaning or add to the context of the sentence or something of that nature.
But when Jesus says this:
31“It was also said, ‘Whoever divorces his wife must give her a bill of divorce.’
32But I say to you, whoever divorces his wife (unless the marriage is unlawful) causes her to commit adultery, and whoever marries a divorced woman commits adultery.
Matthew adds a parenthetical, is he putting words in Jesus' mouth here, trying to explain something that Jesus plainly taught elsewhere so it's not construed the wrong way or is there a way of looking at dialogue that actually includes this. I have a hard time accepting that it's just something Jesus muttered under His breath.
For purposes of translation here's what's on Newadvent.
But I say to you, that whosoever shall put away his wife, excepting for the cause of fornication, makes her to commit adultery: and he that shall marry her that is put away, commits adultery.
Ego autem dico vobis: quia omnis qui dimiserit uxorem suam, excepta fornicationis causa, facit eam mœchari: et qui dimissam duxerit, adulterat.
ἐγὼ δὲ λέγω ὑμῖν ὅτι πᾶς ὁ ἀπολύων τὴν γυναῖκα αὐτοῦ παρεκτὸς λόγου πορνείας ποιεῖ αὐτὴν μοιχευθῆναι, [καὶ ὃς ἐὰν ἀπολελυμένην γαμήσῃ μοιχᾶται].