It's about Ben Gibbard wondering whether he's ready to settle down with a girl for the rest of his life, as evidenced by "I'll change, love, change/I'll change for you/when I'm ready to." And the slurred words in "even slurred words contain some truth" are probably "I love you" and he's wondering if those words do indeed contain truth and he's ready to stick with the girl in question.
It's about Ben Gibbard wondering whether he's ready to settle down with a girl for the rest of his life, as evidenced by "I'll change, love, change/I'll change for you/when I'm ready to." And the slurred words in "even slurred words contain some truth" are probably "I love you" and he's wondering if those words do indeed contain truth and he's ready to stick with the girl in question.