itch.io

To confirm, you mean adding sound captions and not an image caption, correct? While it's not built in to Caption Tool, you can use sound captions in a button action with regular Ren'Py by using the function as an action in your button instead of using activate_sound.

Example:

imagebutton idle "images/mag.png" action [Show("map_mag"), SetVariable("chaptermap", "mag"), Function(play_sound, magazine)] focus_mask True

For this to work, you need to also have your magazine sound registered in your list of sounds.