Next time you're watching your favorite movie, close your eyes and listen to the sound effects. Everything from footsteps to machines to ambient crowd noise is a sound effect.
They were either recorded during shooting or added in post-production. You'd be surprised how integral good sound effects are to movies.
You should give them a try in your own movies or slide shows. You can find thousands of free ones at AudioMicro.
System: Windows XP, Vista, 7, Mac OS X