* "Their salary" is correct if you are referring to the salary of a single person, even if you are talking about multiple people. For example: "Their salary is $50,000 per year." (This means that all the people you're talking about earn $50,000 each).
* "Their salaries" is correct if you are referring to the salaries of multiple people. For example: "Their salaries are all different."
In short:
* Use "salary" when referring to the income of one person.
* Use "salaries" when referring to the income of more than one person.