On Mac the role of a JFX non-editable combo box maps to a Mac role of Pop-up button. When up/down is pressed on the native pop-up button a menu pops up and the selected item is focused and spoken by VO. When up/down is pressed on a JFX non-editable combo box there is no popup and nothing is heard. It's OK that there is no popup in response to the arrow but not hearing anything is an issue.

Test this using java -cp <path>/Hello.jar hello.HelloComboBox.