"Alices" could either mean more than one Alice, or I guess it could be a name, though I've never heard it.
"Alices' " would only work if something belonged to multiple Alices or belonged to someone named "Alices"
"Alice's" concievably could be used as a contraction of "Alice is," but it usually isn't. It would usually be in reference to something that belongs to Alice.
However, with a word such as "your" or "its," it is possessive without the apostrophe. With one, it would be a contraction of "you are" or "it is" (of course if it were a contraction of "you are" it would be you're, not you'r.)
If you're talking about grammar nazis or CDs, you do not use an apostrophe, even though everyone does, unless you are using it as a possessive. Plurals do NOT use an apostrphe, unless it is a combination of a plural and possessive, in which case the apostrophe comes AFTER the plural "S" and is NOT followed by another "S."