Flyer and Cabinet images are stored in the Folders "Flyers" and "Cabinets" in the Folder where the gam db file ( gamesv19.0.0.db ) is.
When you drag/Drop in manager the file is copied over to the folder above, and stored as a jpg if that is selected in prefs.
Default sounds are where the default sound effect files are stored ( Button down, coin mech etc ).
Masks are the default lamp masks that can be used when setting up lamp masks using the right click "Default" option.