When can players not hear a bard's performance?

I'm talking about Pathfinder in particular, but it probably works for most games with a bard. I don't think I've ever played a game where the DM has ever even mentioned if the players could hear the performance, but that is really the only stipulation on if the buffs apply.

It has always just been assumed that if the players are sort of close to each other, they get all the buffs, but is that the way most people play it? Is that the way it should be played?

For instance, if the PCs are fighting in a war LotR style, with giant bellowing monsters and thousands of people fighting, would some GM call need to be made on who can hear what?