It struck me that in Soul Music, Pratchett in some ways satirizes the excesses and madness of "modern music" whether rock & roll or the like. But he also acknowledges that MUSIC is a magical force that can move the emotions in ways that are otherwise inexplicable. I am thinking, for example, not just of the band's concert, but of Buddy's last solo piece that transports all who hear it to a forgotten (if different) place--a home that brings warmth and joy. Then after only one book, he again turns to music, and once more spends much more time parodying it's excesses than examining the nature of "serious" music such as opera.
But opera is essentially Music, with singers who move around and tell a story. But the important thing in opera is the Music. And singers, like athletes, have to have certain types of physical equipment (extraordinary breathing capability, unusually fine vocal cords, etc.). And the people who write operas write MUSIC--not the "play" which provides only a framework on which the music appears. Last night I watched an incredible performance of Bizet's Carmen on Great Performances at the Met featuring a young new soprano, Elīna Garanča. It's not a role that Agnes could have played--especially as Elina emphasizes the sexuality and dancing of the major character in dazzling fashion. But it is the music which drives the production.
Pratchett, in this novel, gives some acknowledgment of the power of music (which is after all only separated from magic by two letters), while he tries to blend a satiric comedy with a mystery. But operas present love, greed, passion and death. In the case of opera, it is the music which elevates them while in other hands it can be the written language (which sings in it's own way) that stirs the emotions.
All of this goes to why it doesn't make a great deal of difference what the soloists of opera look like physically. What really matters is their voices. And the one time that Agnes really gets to sing an aria (to Dr. Underwood and accompanied by Andre), she leaves them both "blown away". That is real star quality. What Christine has is sex appeal and nothing more.
All of which has relatively little to do with the strengths and weaknesses of the book. I think that quite possibly the problem with the novel is that Pratchett tried (as he did in SM) to combine light comedy with an undeveloped exploration of the nature of the criminal and the suffering he causes to others because of his self-absorption. Pratchett will get much , much better at dealing with this kind of blind fanaticism (whether quasi-religious or simply greediness). That is why Thud!, which has some marvellous comic elements, is so much better than this book.