Motif: a dominant or recurring idea in an artistic work; a melodic, rhythmic, or harmonic cell; the smallest independent particle in a musical idea.
The soundtrack presents the listener with two very similar variations on a single motif: first, in The Music of the Night and shortly after in All I Ask Of You. An investigation into each yields the following:
Building up to a "five-to-one" cadence, i.e. going from a dominant/5th chord to the tonic, is the most common way to create and resolve tension and evoke emotion through music. Even without instrumentation, the cadence is strongly implied by the motif itself, as its two strongest notes apart from the final root note are the 5th and the 2nd, both being notes that are part of a dominant 5th chord.
This section goes through all instantiations of the motif and what kind of emotion it might represent in each case.
This performance is pure romance. Erik puts his best foot forward to seduce Christine: he is dressed up and all candles are lit as he takes her into his domain. The music embodies hope and triumph, with no resentment shining through – here Erik is the Angel of Music and not the Phantom of the Opera; he is passion without fury.
The climax is properly performed and follows expectation, but is weak. Musically, the most expressive passage (Let the dream begin / Let your darker side give in) takes place on a subdominant chord (neither tense nor resolved) shortly before the motif is sung, causing the latter to have less of an impact.
A romantic duet, honest and hopeful but lacking passion. Raoul is a good man who cares about Christine but he comes off as a pathetic, pleading dog. The instrumentation is equally romantic but lacking in dynamism. The motif is performed in a beautiful harmonization but is overshadowed by the pompous brass and string orchestra.
Erik has candidly watched Raoul and Christine's love dance on the roof. The reprise here is musically minimal and Erik's frustration, anger and jealousy is expressed almost purely via the motif, which in this case ends on a parallel minor chord instead of the expected major chord. It's a cathartic moment but wholly one-sided; breaking through a wall, but into hell.
The next act begins.
Setting off the movie's final act, the most potent buildup so far in the movie is completely denied to us as Christine pulls off Erik's mask before he can sing the final note and complete the motif.
So far, the listener has been presented with the motif at least four times but never given a full release, no truly cathartic experience.
Apart from being too early in the story and having a purely negative color, the climax in All I Ask of You - Reprise lacks balance; it loses power by overshooting from passion into hate and has too short of a buildup.
The climax in The Point of No Return is set up to be perfect; a six minute long forbidden dance of passion between Erik and Christine culminating in a strongly expressed motif that is cut off at the last note.
In The Final Lair, the story has progressed further. Erik has fought Raoul and would have killed him had Christine not given Erik the first kiss of his life, whereupon he lets them both go.
I will leave the interpretation of the ending up to the watcher/listener.
Looking at the delivery of the final instantiation of the motif, one sees that it achieves what all previous presentations could not. It follows an extended, dynamic buildup; it expresses finality and embodies a new passion – not triumphant as in The Music of the Night, not performative as in All I Ask of You, not hateful as in the latter's Reprise and not desperate and unstable as in The Point of No Return, but one of pure and lucid catharsis.
Musically there is a strong emphasis on the 5th and the 2nd, implying an dominant 5th chord that wants to resolve to a major tonic, which it finally does, completing the final act.