Audio problem: At dinner as they all listen to Colonel Kurtz' voice there's a close-up of the tape recorder and you can see that his voice is overdubbed because the level meters are not moving as he speaks. (00:13:30)
Audio problem: Sitting on the boat listening to the B-52 strike, when Clean says, "Charlie don't never see 'em or hear 'em, man." his lips do not match his words. (00:24:20)
Audio problem: As they look at the B-52 strike from the boat Clean says, "Secondary burning," then he says something else that was edited out. (00:24:25)
Audio problem: Right when Captain Willard introduces himself to Colonel Kilgore, the audio track for the background battle sounds cuts out for few seconds. (00:27:20)
Audio problem: When Colonel Kilgore offers the wounded VC soldier a drink of water from the canteen he says, "any man that's brave enough to fight…" and his lips don't match what he's saying. (00:29:30)
Audio problem: After they all get into the helicopters and take off, a soldier is blowing "charge" on his trumpet. This is an obvious over-dub, as he takes a deep breath in the middle of it without missing a note. (00:35:10)
Audio problem: On there way to Vin Din Dop the music for "The Ride of the Valkyries" is an overdub as the VU meters do not move when the tape recorder is running. (00:37:25)
Audio problem: In the first aerial shot of the schoolyard the closed captioning reads, "this should make everything burn" but if you listen closely what the pilot is actually saying is, "can somebody cover that downed bird". Also the helicopter he's talking about hasn't been shot down yet. (00:40:20)
Audio problem: Just before the flare flies into Colonel Kilgore's helicopter, Lance says, "Nice shot, Bill" but his lips are not moving. (00:41:50)
Audio problem: On the beach Colonel Kilgore takes the radio phone from one of his soldiers and calls in the napalm attack on Vin Din Dop. If you look closely at his lips they do not go along with what he's saying. This happens again two shots later when he sends the Vietnamese woman and her baby to the hospital by helicopter. (00:47:25)
Audio problem: As Captain Willard walks Lance away from Kilgore you hear him say, "just keep walking". If you look closely at his lips you'll see he's saying much more before that which is dubbed out. (00:50:45)
Audio problem: Colonel Kilgore takes the megaphone and says to the surfers, "Okay give it a try guys, one goes left and one goes right "and his lips do not match what he says. In these several shots he's saying a lot more than what we hear. (00:50:50)
Audio problem: On the boat after they offer the joint to Captain Willard, Clean says, "All right, that's great" and his lips don't match what he's saying. (00:52:40)
Audio problem: Before they dock at the USO show at Hau Phat Chef says, "Jesus". He also says some other stuff that has been edited out. (01:04:00)
Audio problem: After Captain Willard grabs the sergeant by the throat and pulls him over his desk Lance says, "Dog starts barking" and his lips don't move. (01:05:35)
Audio problem: As Bill Graham comes out of the Playboy helicopter he takes a microphone and says to the audience, "how ya doin' out there, I said how ya doin' out there?" and his lips are not in sync with what he's saying. (01:06:35)
Audio problem: As Playmate of the Year Carrie Foster is brought out to the audience carried by two Green Berets watch the band behind her closely. The bass player stops playing as he scratches his head but the bass on the soundtrack keeps going. (01:07:55)
Audio problem: We hear an over-dub of Playmate of the Year Carrie Foster shooting her cap guns three times. But if you look closely there's smoke only the first two times and the third time she doesn't actually pull the trigger. (01:08:40)
Audio problem: As he leaves the USO show Bill Graham ignites a smoke canister before he gets into the helicopter and says, "So long," but his lips indicate that he's just whistling. (01:10:10)
Audio problem: We see and hear Chef rip the picture from his Playboy magazine of Miss December in half. In the next shot as he tapes it up behind Chief Phillips it's all in one piece. (01:12:05)
Answer: It's not any specific song; it's just the kind of generic piano music you'd hear at dinner at a high-class restaurant of the era. Think of scenes in movies, films, etc. set or made in the '50s and '60s, where the characters go to a nice dinner and there's someone at the piano playing unobtrusive music to accompany the food/conversation.