Seriously, I always thought that Aragorn could ask the dead to fulfil their
oath to the extent of their original mission, but couldn't ask for more.
I like NZ Strider's explanation much better and it's probably much in line with
Tolkien's
intent.
Then with a crash came a great ringing shout: ra-hoom-rah! The trees quivered and bent as if a gust had struck them. There was another pause, and then a marching music began like solemn drums, and above the rolling beats and booms there welled voices singing high and strong.
We come, we come with roll of drum: ta-runda runda runda rom!
