According to what I know about online novels, the trailer of "Journey to the Green Hairpin" is indeed presented in the original voice, and the main film may also maintain this feature. It's up to the studio. If they want to keep the scenes and atmosphere in the trailer as much as possible, they might use the original sound in the main film. On the contrary, if they decided to use soundtracks or other methods to enhance the effect of the movie, the original sound from the trailer might not appear in the main film. However, regardless of whether it was the original sound or the background music, the film production company would usually keep two different sound styles in the trailer and the main film to ensure that the audience could hear the two different sound effects without confusion.