What is the best place to use audio on a Website?
Just about anywhere! However, there are some things you should take into consideration. In many cases "less is more." That is, the less audio you use and the more effectively you place it, the faster your site loads and the more impact your audio or voice talent has on the visitor.
I like sites that get creative with rollovers (audio plays when you move the cursor over a button) and ones that have a good quality "intro." Sound effects are nice, but a blind person will not know what the button is if you don't have it voiced or your "alt" tags are wrong.
Other uses are Flash presentations which load all the graphics and audio at once. Check out this page for some really cool Flash presentations I've recently voiced.