If you do not use your own _sound.* files then you inherit the mapDE/mapUS environmental sounds like birds, water, wind, etc. See below how to add custom _sound.* file setup to your terrain.
- copy mapDE_sound.i3d and mapDE_sound.xml files from fs19_root/data/maps/ directory to your terrain maps/ directory
- rename mapDE_sound.* files to your terrain name like name_sound.* (one of ours is PMC_Iowa_Ringsted_8km_sound.* so its not "name" literally, its just for examples sake)
- GE edit name_sound.i3d and remove the sounds you dont need, you can also move or add sounds
- edit name_sound.xml and change filename="$data/maps/mapDE_sound.i3d" to filename="maps/name_sound.i3d"
- edit maps/name.xml and change <sounds filename="$data/maps/mapDE_sound.xml" /> to <sounds filename="maps/name_sound.xml" />
All done, now your terrain will play the sounds from your maps/name_sound.i3d file, so no more ocean sounds in middle of the terrain in fields etc (assuming you removed the water sounds).