The Walls of Jericho
The most obvious and most frequently used symbol in the film is, of course, "the Walls of Jericho." By the end of It Happened One Night, all three of the movie's most important characters—Ellie, Peter, and Andrews—are in on the joke, which Peter is the first to devise when he puts a blanket up to separate his side of the cabin from Ellie's during their first night together.
The actual Battle of Jericho, as recounted in the biblical Book of Joshua, is a pretty dark and disturbing episode. But Peter makes light of it when he says that the blanket he's stringing up will be indestructible, like the Walls of Jericho. He's probably unaware of the irony that, in the end, these walls were totally brought down by a lot of noise.
Trumpet-blasts are traditionally credited with this feat of destruction, and that's why, in It Happened One Night, the last thing we hear about is Peter's request for a trumpet. The wall that separates Peter and Ellie—and that seals his promise not to violate her space or her body—finally comes down. Which means these two are going to have at it to their hearts' content.