You can learn a lot by listening, as Todd Decker, professor in music and film and media studies, shows in his book “Hymns for the Fallen,” a look at post–Vietnam War Hollywood film sound. Films such as “Apocalypse Now,” “Saving Private Ryan” and “Platoon” use sound — dialogue, music and sound effects — to let audiences encounter what the modern American soldier went through in war, in an experience both patriotic and memorializing.