Its really easy if you have sfx and music WAV files. Just right click on either the Music or Sounds folder and import the WAV, then simply add them where necessary. Be sure to include the Audio plugin, otherwise you won't see any options to add the music/sfx to events. I'll make 2 simple examples here:
For Background Music
EVENT: System > On start of layout
ACTION: Audio > Play > select the music file, change to looping and leave the rest as it is
For Fireball Sound Effect
EVENT: Find where you have the player clicking and creating the bullet Fireball
ACTION: Add a new action > Play > select the sfx, leave the rest as it is
Hope this helps :)