An example of aural comprehension of plot action. Harmonica once again makes his entrance first on the soundtrack then in image. His harmonica playing is his call-sign. The strange sound is the gun clicks prior to the shoot-out explosion. As Harmonica climbs down off train roof and comes face to face with Frank's gun, the harmonica motif is sounded - this time without Harmonica visibly playing it.
Sounds often signal death by rupturing silence, giving away cover and marking presence.
Note also that two gun shots happen simultaneously, letting Frank think he shot the fat man when in fact both he and Cheyenne did. Each time a name is sounded, the distant harmonica sound appears, symbolizing the slight triggering of Frank's conscience-free memory. In the end, he dies by remembering his past, while she wins by forgetting her past. Words signify explanation - which is why hardly anyone ever says anything, otherwise the whole plot would finish. The unsaid mobilizes the action.