The running time of 2 hours 28 min is a reference to the original length of Édith Piaf's song "Non, je ne regrette rien", which lasts (on its first recorded edition) 2 minutes 28 seconds.
The slow, gloomy, blaring trombones in the main theme of the film score are actually based on an extremely slowed down version of the fast, high pitched trumpets in the beginning of the Édith Piaf song "Non, je ne regrette rien," which is used as a plot device in the film. Furthermore, when music is heard by someone who is currently within a dream, the music is perceived as slowed down. Thus, the main theme of the film score is almost exactly what the beginning of "Non, je ne regrette rien" would sound like to a dreamer. This thematic device is brought to its logical conclusion when the song plays at the end of the credits, signaling that the audience is about to 'wake up' from the film.