Why can't James, Laurent & Victoria "hear" Bella's heartbeat as they are standing in the field? In all of the books it is made quite clear that the vampires hear Bella's heartbeat whenever she's in close proximity to them. Any ideas?
There isn't any way of providing a canonical answer without asking the author. However, I would presume that it is probably more a question of what they would notice versus would they could hear if they were concentrating.
For Twilight Universe vampires, smell is one of their most critical senses, especially when hunting. The Cullens go to considerable effort to keep JLV far enough away from Bella to not smell her and, at that distance, the sound of her heart beat would be extremely faint, especially compared with the sounds of nature in a thunderstorm.
This is now addressed in Midnight Sun. It's revealed that Jasper was projecting his power to control emotions to make Bella appear "unimportant." Unfortunately, the camouflage is not strong enough to counter the gust of wind that blows Bella's scent towards the visitors.
From Midnight Sun Chapter 22:
Jasper was concentrating so hard that, had he been human, his body would have been dripping with sweat.
I’d never seen him do this before, or even guessed that it was possible. Was this something he’d developed during his years in the South? Camouflage?
He was concurrently smoothing the tension surrounding the newcomers and making anyone looking in his direction feel singularly uninterested. Nothing could be duller than examining this nothing male at the back of the group, so unimportant.…
And not just him… He was covering Alice, Esme, and Bella in the same haze of tediousness.
This was why none of them had realized yet.
Edward is also tapping his foot in rhythm with Bella's heartbeat, but the implication is that this was not as strong a camouflage as Jasper's power.
From Chapter 21:
I shifted my position so that my body would hide hers and began tapping my foot quietly to the rhythm of her heartbeat, hoping to disguise it as long as I could by creating a plausible source for the sound.