sound effects software?

does anyone know of some free software that can add sound effects to my blender animations?

Audacity may be something for you. You can add effects and edit the sound. Most of the page is in swedish it seems so you can download the latest version here.

Also checkout the freesound project, lots of sound effects there
http://freesound.iua.upf.edu/

and you can get good music and mixes, also creative commons licence from ccMixter

Watch out for the CC licence as anything created is therefore subject to the same license. So if you use other people’s models…they may not be compatitble with CC. As a general rule, I tend to avoid CC for video/audio projects because there can be problems.