Video backgrounds
Give me free art, a site where you can download animations, textures, backgrounds & wallpapers, photos, web templates, icons and logos, you can even ask the person behind all this to make stuff, sometimes he will do it for free, other times you'll have to pay him, this all depends on what you're asking of course :) .
Video extra sound effects
Universal soundbank is a site where you can get lots of free sounds, they are all organised by the alphabet. You can hear them before you download them, if the video part doesn't work, you just have to click on the same name on the bottom of the page, then you can hear it too. These sounds are all free.
SoundBible, a very good site (in my own opinion), you can search for what you need by typing in a keyword in the empty space on the right. After you clicked on the little symbol, you will be brought to a list of sounds which the website linked to the word you just typed. You can listen to them on the same page and when you want to download them, all you have to do is click on the green words, and then click on the picture of the file you want to download, the files are supported in wav, mp3 and zip-format.