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That IS the folder I"m trying to put the sound in, but when I do an error comes up saying "this can't be modified" You could change the error sound in system 9, why not in 10?
 
You also might try the one under your home directory.

The only reason I can imagine that it wouldn't work is that permissions aren't correct for the folder. Mine shows that System owns it, which means the regular administrator can't get to it. I wouldn't know why that would be important to keep safe.
 
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