While I agree that context is important, I do not believe it is always external context. Without an external context to indicate otherwise, I would assume the second sentence is what was meant, not the first.
The reason why is that, at least in my variety of English, "may" generally defaults to its permissive sense if possible. Since "slurp" is an action, and one can conceivably be given permission to "slurp," the permissive sense makes the most sense. In order to convey the possibility sense, I would use "might," or, more likely completely recast the sentence, such as "Be careful not to slurp your soup."
Examples for when "may" cannot default to its permissive sense would be in such constructions as "may [not] be," "may [not] need," or even "may [not] want."