One of the most intriguing things I've learned while reading (and reading about) Buddenbrooks is that it was supposedly Faulkner's favorite novel. (This is an unattributed statement in the book's Wikipedia entry, yes, although I've encountered the sentiment elsewhere.)
Granted, Lowe-Porter's translation of Buddenbrooks didn't appear until 1924, so I can't say if Faulkner read it before or during his work on The Sound and the Fury, but I'm willing to believe that's the case based on some similarities in themes and characterizations. In particular, the relationship between Tony and Tom in Mann's novel seems like an influence on that between Caddy and Quentin Compson. (That would leave poor Christian as the Benjy character, a comparison that holds less water but can still be made.)
Tom and Quentin share an inordinate level of personal responsibility and self-identification with their respective families, a situation that leads in both cases to mental instability. But the really interesting parallel is the extraordinary sympathy that both Mann and Faulkner show towards their female protagonists. Tony's horribly flawed marriages both occur because her being married is considered beneficial for the family business. Faulkner shows similar care in exhibiting how Caddy's "purity" relates to the Compsons' skewed, outdated sense of pride. In both cases, the women function as symbols of the families' false belief that they can control human relationships, particularly as generational shifts change the societal expectations of what's acceptable in those relationships. I'm curious if anyone has any further insight regarding Faulkner's reading of this novel, particularly when his first encounter with Buddenbrooks occurred.
Similarly, after reading Mann's beautiful rendering of Hanno's musical experiences in Part 8, I'm interested to learn more about the role of music in the writer's life. Scott, having enjoyed Doctor Faustus so much, perhaps you've read about Mann's musical background. Was he a musician himself, or was his understanding more scholarly? He seems to grasp the unspeakable aspects of musical communication with enough depth that I imagine he at least played as a hobbyist, but I'm not sure.