The plot of Cosi doesn't resonate very well with 21st century folks. The attitudes towards relationship and "love" and issues of a woman's being free to pursue her own calling and destiny are now all terribly removed from the upper class Vienna of the late 18th century. The fact is that the enlightenment had emerged from a long period of time when women, especially upper class women, were commodities and property to be disposed of by the men in her life. Descartes and others presented a view of the place of women that raised them up on to a pedestal as the sacred guardians of virtue and family. It is at this time that the whole distinction between virgin/whore emerges - not that it wasn't there before but it becomes a stronger emphasis. The whole point is that a woman's life was considered incomplete if she were not married with a family. This is true of men too - marriage to a faithful, loving wife was the entire point of life. Look at Mozart himself, marriage at all costs. He was so desperate to get married he married the sister of the woman he wanted to marry. It is important to note that the women themselves of the time embraced this, as it was an improvement over being pawns of their fathers and brothers (not that this was completely eliminated). There was also more of an emphasis on "love." But it is hard to distinguish love for lust, or love from the fear of being alone. This all explains the plots twists of Cosi: the guys are certain of their girls devotion and fidelity, but they don't know them at all since getting to know the girl was really irrelevant, all they wanted was to get married to these perfect female persons. So the women start out on this high pedestal. Alfonso is simply trying to bring the men to the point where they see that the girls are human beings, flawed human beings just like them. For their part the girls are terrified of ending up alone. For a more detailed discussion of this read: In Defense of Cosi.
As far as musical clips: the trio "So soave il vento" is perhaps the most sublime and gorgeous of Mozart's works. Also, I would point out that from an orchestration standpoint there is an emphasis on the winds, which are very prominent throughout. The duet "Ah guarda sorella" is interesting (note the use of the clarinets) for at this point these girls have no personality, they are bored and they are like dolls. Only when the boys leave do they come to life. I love Dorabella's recit "A acostati," which is followed by her wonderful arietta "smanie implacibili" - note the weeping figures and the desperation. For my money Isabel Leonard totally captured this aria in her HD performance a year or so ago. Then there is Come scoglio - again, listen for the winds - Mozart is like a kid with a new toy. Certainly he uses the winds in DG and Nozze but not like in Cosi. The first act finale is tremendous and a great example of Mozart's now well worked out ability to create magnificent ensembles - I particularly love the part where they wake up and wonder if they are dead and in heaven then the fast finale. Great!
Act 2 begins with a lovely arietta by Despina - "Una donna a quindici anni." Note how Mozart keeps extending it, it never wants to end, and again the use of the winds. Then, once the girls decide to give these "Albanians" a hearing we have the "date" and what does Mozart use - winds - clarinets, horns and bassoon in ensemble, to which he eventually adds the chorus - "Secondate." Then Guglielmo manages to break Dorabella in the gorgeous duet "Il cor vi dono" (again, I can't say it enough - listen for the winds - specifically the clarinets and bassoons - incredible orchestration - this duet makes me cry, it is so beautiful, especially when they notice their hearts beating! Then poor Ferrando strikes out with Fiordiligi and she sings Per pieta - we now have a featured horn solo! Also, compare and contrast come scoglio with per pieta - "Like a rock!" but now she is completely exposed and vulnerable, this is why the horn is such a perfect instrument as it all feels so vulnerable (it is also wicked hard for the horn). Also listen for the clarinets, bassoons and especially the horns at the very end. Then the boys come together to lick their wounds and Guglielmo sings that cute arietta complaining about women "Donne mia, il fate a tanti," (winds) but it is hard to be sympathetic with him at this point (In the clip that is linked Thomas Allen sings the arietta as an angry diatribe, which I think is exactly right). I don't think Mozart was, this is why this arietta almost sounds like Papageno, and Guglielmo sounds like a whiner. Also, unlike Nozze, where the arias complaining about the sexes are balanced (Figaro complaining about women and Marcellina complaining about men) this arietta is not deserving of a valid response, he has no valid complaint and deserves everything he is getting. Then we are into the finale of act 2 - which ends under a cloud - especially Guglielmo cannot come to grips with his anger, and Ferrando just says that he is never going to question fidelity again, it isn't worth it. Ultimately there are no happy endings for these couples. But it is important I think to note that it is not the women's fidelity that has been called into question, it is the attitudes, perhaps the entire worldview of the two boys. And this is more serious, Ferrando I think can move forward, Guglielmo, not so much. Finally, I did not like the way the Met HD production had Despina react at the end - it is obvious from that that ultimately the director did not get this plot. I think Despina knows and is completely complicit throughout.
Cosi is a masterpiece - both musical and the libretto. Enjoy this complete performance of the opera: