I thinnk the smallest audio files are wma, which Windows' very own media player can convert for you, I think. Not 100% sure on that though so don't quote me.
DivX is the most commonly used compression for hosting video files on web sites, the DivX site has converter software, it may be free but I think you may have to pay for it.
I use software from a company named Imtoo (Google it) for all those functions.
And Irfanview (free) can resize pix, as can Google's Picassa software (free) I believe.