Mark K

If the LOTR soundtrack had to have been filched from existing classical music, my top suggestions would be:

The Shire:
Beethoven's Pastoral Symphony (1st, 2nd or 5th movements), with Bilbo's party characterised by the 3rd movement 'Peasants' Merrymaking'

Another theme for the four hobbits early in the Fellowship :
The fourth movement of Prokofiev's Classical symphony, fast and carefree.

The Black Riders:
'Dies Irae' from Verdi's Requiem

Isengard:
'Mars' from The Planets, in the same 5/4 time as Howard Shore's theme

Fellowship's trek south (where Gandalf leads the way)
'Jupiter' from the Planets (Holst), the slow middle section

Gollum's theme:
The twisted and sly 'The Gnome' from Mussorgsky's Pictures From an Exhibition.

Frodo and Sam's journey in Mordor and the destruction of the One Ring:
The March to the Scaffold, from Symphonie Fantastique by Berlioz. The music becomes violent towards the end (Frodo and Gollum ?), there is a piercing oboe theme (Gollum's last 'Precious!') and the whole ends in a drum roll (the Ring's destruction).

The battle of Helm's Deep, given its 'Nordic' character:
'Finlandia' by Sibelius would be perfect, with its triumphant finale.

The Battle of the Black Gate:
The third and fourth movements of Beethoven's 5th symphony would be good there, as Sauron's doubt could be marked by the switch from minor to major key, and the collapse of the Barad-Dur the magnificent finale.

The Hobbits' return to the Shire:
The second movement of Dvorak's New World symphony (the oboe solo), appropriately based on the folksong "Goin' Home"