To make the sound of chanting orcs, Peter Jackson used the sound of cricket fans in the Two Towers.

To make the sound of chanting orcs, Peter Jackson used the sound of cricket fans in the Two Towers.

In an international match between New Zealand and England, fans were offered the opportunity to make a cameo appearance (in audio form only) for the upcoming Lord of the Ring film. Peter Jackson and his sound technicians appeared at Westpac Stadium in Wellington, NZ to lead a crowd of 20,000 rowdy cricket fans in chanting, screaming and grunting. The sound would later be used for the Uruk-hai preparing for the battle of Helms Deep.

Another fun fact: Peter Jackson makes a cameo appearance himself in the very same battle, throwing a javelin into a crowd of Uruk-hai. Also, for some reason, this fictional battle has its own Wikipedia page.
(Sources: 1, 2)