A setting is being described here. Pastoral sounds like pasture, which is a field where animals graze and stay. When they are “burned by the setting sun” they aren’t actually burning, but the sunset, which is usually orange or yellow in color, is reflecting onto the field.
The peaceful noises of the farm are the noises of animals as they are getting ready for bed.
Pastoral fields are fields in which farm animals are raised. When it says it is "burned by the setting sun" it means that the colors of the sunset are sort of reflecting off the grasses in the fields, making them look golden, as if they are on fire.
