As the rather dominant narrator of both of these stories, Buddy doesn't leave much space between himself and the author (Salinger, that is). In other words, Buddy's tone is pretty much Salinger's tone there. In other words, there's no space between the author and the narrator where we might examine the differences between them or consider Salinger's attitude toward his own creation (Buddy). This isn't true of all first-person narratives, but we think we make a good case for it here. (Feel free to disagree – we're all ears.)
In fact, some have even argued that Buddy is Salinger's alter ego. Some critics argue that Salinger speaks entirely through Buddy, that Buddy's ideas are Salinger's own, and that Buddy is little more than a pen name with a few fictional attributes. (It helps that Salinger gives Buddy his own year of birth, 1919, and that he actually calls him his "alter-ego and collaborator" on the dust-jacket of Franny and Zooey.) Because of the closeness between Salinger and Buddy, we're going to take some liberties in this section on authorial tone. While "Tone" is defined as the author's attitude towards his subject, we're going to look at Buddy's tone in the course of these two stories.
You probably laughed out loud several times in the course of reading "Raise High the Roof Beam, Carpenters." Buddy's has a particularly wry, incisive, and observant sense of humor. When Buddy helps the wedding guests into the car "without any degree of competence whatever," he admits that he is "not only twenty-three but a conspicuously retarded twenty-three" ("Roof Beam" 214). When he cracks his head on the roof of the car, he refuses sympathy and explains, "I was the sort of young man who responds to all public injury of his person, short of a fractured skull, by giving out a hollow, subnormal-sounding laugh" ("Roof Beam" 2.15).
"Seymour" harbors its own share of hilarious self-deprecation. After having failed to establish a plot or any kind, Buddy parenthetically admits, "Buddy Glass, of course is only my pen name. My real name is Major George Fielding Anti-Climax" ("Seymour" 1.38).
But we know for a fact that modesty isn't one of Buddy's virtues. He doesn't make fun of himself in a bashful or embarrassed way. He laughs at himself the way only a truly confident person can. That's why we call his attitude mock self-deprecatory.
We should remember of course that, for all its humor, this book is far from a comedy. What makes its subject matter so poignant is the combination of this true-to-life humor and the very palpable emotional guts of the Glass family saga. Let's not forget that Seymour kills himself only a few years after the wedding, and that Buddy is narrating the story of the wedding day after Seymour is already dead. Perhaps the best example of this bittersweetness is the paragraph in Seymour's diary about the scars on his hands "from touching certain people" ("Roof Beam" 4.13). As a reader, you get a two-fold reaction here. First comes the smile, because this is a touching and adorable sentiment. But it's also incredibly painful – the word "scar" in itself implies pain, and this is an indication of Seymour's instability, paranoia, and a possible manifestation of Posttraumatic Stress Disorder.
"Seymour: an Introduction" takes us a little closer to the bitter side of the bittersweet, since Buddy is dealing quite directly with the pain of his brother's death. Additionally, Buddy's arrogance, superiority, and cynicism are more palpable in "Seymour" than they were in "Roof Beam," perhaps because he's writing several years later (he does, after all, reiterate several times that he is forty now, and that middle-age has a major effect on his perspective). Buddy's description of poetry critics, English students, academics, and even the general reader is critical and a bit cynical as well.
And yet, we've got the ending of the book to contend with. At the end of "Seymour" Buddy abandons his cynicism in light of an essentially optimistic outlook. What's so interesting about this concluding tone is that its optimism is not a youthful or idealistic optimism. Buddy is fully aware of the limitations of the world and the people in it. He chooses, in spite of everything, to love them and help them anyway. It's a knowing optimism, an optimism accompanied by wisdom.
In any case, make sure you check out "What's Up With the Ending?" for a full discussion of this switcheroo. In a nutshell, however, what we see is that Buddy's cynicism has been a bit of a show for the reader. He'd like to talk the talk of a cynical writer, but the fact is that he loves all those critics and editors and "academicitis"-ridden students after all. See a parallel with "Roof Beam" here? If the tone in the first story is mock self-deprecation, then the tone in the second is mock bitter.