The Inspire Courage bard ability reads, in part:
Inspire Courage (Su): A 1st-level bard can use his performance to inspire courage in his allies (including himself), bolstering them against fear and improving their combat abilities. To be affected, an ally must be able to perceive the bard's performance. An affected ally receives a +1 morale bonus on saving throws against charm and fear effects and a +1 competence bonus on attack and weapon damage rolls[...] Inspire courage is a mind-affecting ability. Inspire courage can use audible or visual components. The bard must choose which component to use when starting his performance.
If you have a bard with Inspire Courage ability atop a castle in an area of perfect acoustics (and/or lighting [think illusions or shadowplay]), could the bard Inspire Courage within an entire army that could see and/or hear him?