depends on the format of your music, but you would use either :
the "sound" command (dbLoadSound() etc) if your music is in .WAV format...
or
the "music" commands (dbLoadMusic() etc) if your music is in .MID or .MP3 format.
you can check for the keypress in your main loop, using the "input" command "int dbShiftKey()" which will return an integer value of 1 if the key is pressed or 0 if it is not..(i dont think that is the ideal or best solution, but it will work for your purposes)
once you have checked for the keypress and it has been detected, calling the appropriate music/sound command to stop the music from the "sound" or "music" command sets(depending on the format of your music as above)
Changing the image is just as easy.... have a look through the help file that comes with DarkGDK, most of the commands are prety self explanatory just by their names, but the help file also contains some useful usage information.
Its best to work out the actual code yourself, as you will learn the techniques better that way.
(however if you still have trouble, post your code here and im sure someone will be able to help you sort it out)
Good luck.
If it ain't broke.... DONT FIX IT !!!