Animals don't just call it a day when the sun sets, most of them are even more active at night.
This is going to have some impact on them, I am sure - someone who knows more can correct me if I am wrong, but I believe the animals don't actually "live" where you view them. They live off-stage, which is where they are used to going at night for food/sleep/etc.
So in order to be on display at night, presumably they are going to be putting in longer hours on the range, when they used to already be "home" and relaxing. So yeah, I can see toning down the audio a bit. Or just getting rid of it altogether...at least anything pre-recorded.