FAQ

Likely because it is meant to represent a helmet from a fallen soldier on the battlefield.

From weed dealers.

Platoon is based on a screenplay by director Oliver Stone. That screenplay was actually based on Stone's earlier screenplay, Break, a semi-autobiographical account of his experiences and people he knew in Vietnam. Platoon won the 1987 Academy Award for Best Motion Picture.

Yes, and it's probably a composite of several of them. The most famous is the My Lai massacre, believed to have been perpetrated by Lt. William Calley. Oliver Stone quite plainly wrote the scene to be reflective of My Lai and probably many other lesser known incidents of misconduct by American military personnel.

Audie Leon Murphy was the most "decorated" (was awarded the most service medals) soldier that served during World War II. He became quite famous and was a hero to Army soldiers for decades and is still widely remembered to this day. The reference to "decorated" can be misconstrued as medals considered to be "worthless." Far from it, as Audie Murphy was awarded the Medal Of Honor and every other medal for bravery in combat, sometimes in multiples. He was also awarded France's and England's version of the Medal of Honor. Murphy was a small guy but was a wrecking crew against the Germans. He received a "battlefield" commission and got out as a captain. His wounds deprived him of his biggest dream which was going to West Point, so instead he became an actor. He later died in an airplane accident. More can be read about him here.

Yes, it was likely meant as an ironic statement by the tank crew.

...near the beginning of the movie, as the squad goes out for the night ambush and in the end of the movie? "Oh! Susanna", written by Stephen Collin Foster in 1847 or 1848. Lyrics here.

...playing when Chris smokes marijuana with Elias and his crew in their bunker? "White Rabbit" by Jefferson Airplane.

...at the end of the movie? "Adagio for strings", composed by Samuel Barber and performed by the New Zealand Symphony.