First on the subject of orcs and goblins -- they are the same in LOTR, the elves call gobbo's orcs, it is the inspired fault of role players and GW that there is a commonly held view that they are different.

Uruk Hai are of course completely different being half men and half orcs.

Goblins (or orcs)

In The Hobbit, they are not immediately recognisable as evil -- but are thoroughly unpleasant and untrustworthy. Their behaviour is more akin to a latter day dictator in central Africa: looking out for an easy life with a bit of torture thrown in on the side.  They are ruled by a fat lazy king (the similarities with central Africa again!) - organisationally their armies could be described as warband but I would prefer militia, fyrd, ashigaru, or bondi. 

In the LOTR film the Goblins certainly demonstrated warband behaviour (as in slaveringly mad, hating, showing no fear - or not much anyway). 

In conclusion I would think that Goblins should be represented by an Anglo-Saxon army which is undergoing a transformation from warband behaviour to a more organised form of warfare (you could argue that warband -- an invention of Caeser, never existed in the first place, but that is boring) - in short a goblin army could comprise warband, fyrd (variously armed), skirmishers, wolf riders (from The Hobbit) and warg riders (as depicted in Two Towers) and trolls - this gives a marvelous varied bunch & difficult to oppose.  Add in Mark’s nice ideas on use of goblin drums and you have a not bad army.

Uruk Hai

No cavalry, some shooters, but mostly either discipline pike, fanatical warband, berserkers (in small numbers) - a la WAB rules, crossbow men (If anyone can afford them) plus siege weapons (long range missiles).  This would be a real nasty army to face.  (well it was a spec. built army by Saruman).

Rohan

It’s tempting to say base them on Normans, but their cavalry behaves like eastern cavalry capable of lance charges and firing bow from the saddle.  I would settle for a "first crusade" army with turcopole cavalry.  This allows discipline foot, rabble, mounted knights/bow armed troops and inspirational leaders.  Foot should include bowmen -- using composite bows (i.e. longbows).  Alternatively a samurai army, early period could fit very well.

Men of Numenor

Tricky, the book implies a highly stratified society with knightly retinue at the top.

Certainly early medieval would appear to fit well - but I think that would allow far too much impulsive knightly behaviour.  I suspect that given the age of the society that either a late period samurai army (without the muskets) or something like the Ordonnance armies would fit best.