I work with PowerPoint a lot, developing training materials for the company. Normal presentations are usually pretty boring, some of the contents are also tedious, so I try to spice it up a little by using film, pictures and sound files. The coolest wav source to check is Wav Central. They have a huge amount of the coolest wav files, the easier to insert in any PowerPoint presentation. Here is one of my favorites, from the Blues Brothers. Another very funny one for smokers. There's also a buch of effects too. These wavs are good for school projects or to customise the sounds in your PC.
If wavs can't be found, they can certainly be downloaded with a little pice of great software: Stream Down. I've been using it for a while now. At first I tried the 15 day trial but then I used it so much that I ended up buying it. Great tool. It works for anything streaming, music and film in a bunch of formats. Can't say how well it works.