Here's a site that has a wealth of sound effects that you can add into your multimedia project: Pacific Digital Media's Free Sound Effects.
They aren't public domain, but Pacific Digital Media has a generous blanket permission for re-use:
I love sound effects. On this site, if you press the play buttons, as opposed to clicking on the text links, you can play multiple sounds at the same time.
Here, if you are at the office, try this: Turn up the volume on your speakers, go to this page of miscellaneous sound effects, and start pressing all the play buttons you can. Have fun freaking out your officemates!
Try starting with cards shuffling, champagne bottle pop, dogs barking, wind chimes, and squeeze toy, with lots of door knocking. Once you think you've got your office neighbors' attention, push it to the limit with "Hair Mousse," "Velcro Strap Pull," "Striking a Match," "Firework Explosion," "Sea Lion 1" and "Sea Lion 2."