To my knowledge there are no sound schemes that are "ready made" on the net - the problem is that the size of the file for all the sounds would be quite large hence the reluctance to make a full package.
There is also the obvious problem that many of the sounds are copyrighted so distribution via a scheme can be tricky legally.
I have designed several sound schemes of my own by downloading individual WAV sounds and then putting them together into one scheme.
There are several sites on the net where you can get them and if you do a search on google or even better do a search on message board topics on Wincustomize you will find some of my past replies on this topic which will give you links to these sites.
I am a great lover of Toon XP and i found a lot of "cartoony" and funny sounds to make into a theme which my little daughter loves (well i must be honest dad loves them even more).
One word of advice - the quality of WAV sounds varies from site to site so listen to them first before before downloading.
One of my favourite sounds is an F16 fighter doing a "fly by". It starts off very quietly and then "wooosh".........it goes from one speaker to the other and slowly disappears. When i first installed that one i did not have the volume level set correctly and it sounded like it was flying through my living room. It was just gone midnight at the time and the wife was in bed and not amused !!!!
Have fun and good luck..........If you can't find the previous threads on this subject give me a shout and i will try and trawl through my files and throw up some links for you.