The RPG-SoundMixer has two basic layouts (Fantasy & Science-Fiction), which refer to the background image. If the current layout does not suit the role playing group, you can simply select another one by selecting "Layout: Fantasy" or "Layout: Science-Fiction" from the "Settings" menu.
You can also include your own layouts. To do so, you have to copy a suitable JPG file into SoundMixer's "Layouts" directory. after doing so, you can select the layout as described above.