You should be able to put the sound files in your local Library folder, found under your home folder. If there isn't a Sounds folder in there, make one.
Apple is strongly recommending that you do not modify anything under System or /Library.
This way, if you have problems, it's really easy to troubleshoot. Just login as another user.
The other reason is so that you can keep your modifications to yourself. Anything in /System or /Library will be there for everyone, so if you put something in there that causes problems, you are in trouble. If you just stick it in your local Library folder, you can always login as another user and fix it from there.
I know I had this problem under Public Beta. I moved some fonts over to my local library folder. Luckily it was my local one and not the global one.