But For Another Three Grand, I'll Look the Other Way
Having reviewed your script as technical adviser to the film, I can say that virtually all of it is historically accurate. To respond to your specific questions: Air strike forces did in fact sometimes play loud music to intimidate the enemy, so this is an acceptable scenario to include; also, it was not uncommon for Green Berets to advance through the military ranks on a timeline similar to that of Mr. Brando's character in the script, and your portrayal of the geography of Cambodia's lower rivers is correct.
I do have one strong issue with a certain line in the film, and must insist that it be changed if you are to claim that Apocalypse Now is a true depiction of the nature of combat. Here is the aforementioned line as it technically should read, according to the facts of battle as I experienced them:
CAPTAIN KILGORE: I love the smell of napalm in the morning. Smells like....cake mix.
Also, may I say, having served in three wars, that I've never heard American soldiers use swear words. We just don't do that kind of thing.
Sgt. Elbin Tatternbuck