If you don't select a directory, a default set of images with german sounds is used. In order to use your own buttons you have to create images and sounds and store them in a directory on your device. The directory can be chosen by a file chooser when first started or after deleting the configuration of the app. You can go forth by the ok button of the app and back by the back button of your device.
The images have to be named button11.png, button12.png and so on (as consecutive sequence). The first digit represents the row and the second the column where the image will be presented. Sounds are named like button11.mp3 and so on. Other extensions for images (jpg and gif) and sounds (wav) are allowed. You can choose between upper and lower case. The name 'button' is fix (in lowercase).
The images are automatically scaled, that all rows are presented on one page. You can add as many columns as you want, they are presented in a scroll view. It's possible to add a special button which switches to the next page when pressed (or to the first page, when the last one is reached). Just name the button for example button13next.jpg instead of button13.jpg.
The app icon can be found at: http://www.iconarchive.com/show/hornet-icons-by-878952/Music-Drive-icon.html
under the license: http://creativecommons.org/licenses/by-nc-nd/3.0/
The file chooser can be found at http://www.codeproject.com/Articles/547636/Android-Ready-to-use-simple-directory-chooser-dial
it's provided under 'The Code Project Open License (CPOL)': http://www.codeproject.com/info/cpol10.aspx
The other icons are from the following pages:
Many thanks to the authors.
Default set with images and sounds (in German) added.