"Sound" means it is already justified--by the facts that
1. it met the requirements for 'validity', and
2. it is a true statement from two true premises (which is not necessarily true of 'valid').
If someone thinks it is not sound, s/he will have to provide the counter-argument; and since you cannot know in advance what that will be, you can only advance your own argument, making certain it is in your opinion 'sound'.
But if what you want is the justification that IT IS sound, you will be giving an explanation about logic more than about the topic of the statement.