You can definitively verify whether this is the issue by selecting your app product under the Products group in the file navigator, right-clicking on it, and selecting show in finder. When you see the app product in the finder, you can right-click on it and request viewing the package contents. Once you are at the root level of the bundle (where your image files are added, etc), if the files were successfully added they will be visible in the app bundle at root level. If they aren’t in there at all, they weren’t added to the app target. If they are in there but they aren’t in the root level (they are inside of a folder that is at the root level) that means that when they are added, the add dialog box settings are incorrect.
I suppose one last option is that it is possible that you are dragging the folder containing the voice data files into a group that already represents a folder within your app bundle. See if you get better results from dragging the folder in to the file navigator right into the project file icon rather than a subfolder.