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.