Hi!
Button sound: In the main_menu screen, find the buttons and after each add:
hovered [ Play("sound", "<filepath>") ]
So for example:
hotspot (1370, 259, 258, 108) action Start() hovered [ Play("sound", "audio/click.ogg") ]
Placeholder image: You'll need an image editing program that supports psd files and/or layers. Some free ones: Gimp, Medibang, Krita. After you got one, simply open the title.psd file included in the gui folder, customize it to your liking then save it to the gui folder as title.png, overriding the placeholder image. You can also just open the title.png and directly customize that.