Correction: "Although the premises are false, the argument itself is valid since it can be inferred from the premises that Superman is indeed AN ACCOUNTANT"....

And thanks for adding the bit about soundness Cant Get Right*

Hey, I'd like to take a swing at this one. Beshir, Let me know what you think.

Essentially, all premises could be false and yet an argument could be valid if and only if the conclusion follows logically, i.e.,

Superman never flies in the comics.

If someone never flies in the comics, then they are an accountant.

Therefore, Superman is an accountant.

Although the premises are false, the argument itself is valid since it can be inferred from the premises that Superman is indeed a farmer. Conversely, a conclusion which states that if one is an accountant then he/she is Superman is invalid since that statement does not follow from the stated premises. If a conclusion is stated which cannot be supported from the given premise(s), then that argument is logically invalid and some assumption(s) underlies it. Additionally, if an argument proffers a conclusion which is indeed in and of itself a statement which is known to true but does not follow from the premise(s) offered in support of it, then, taken together, the argument is invalid.

To reiterate and expand a little: you asked,

"If the conclusion does not HAVE to be true, then the argument is invalid?"

The short answer is yes if you're asking whether the conclusion MUST be true on the basis of the premise(s). Again, if the conclusion does not HAVE to be true on the basis of the premise(s) given in support of it, e.g., if the conclusion is not entailed/cannot be inferred from the premise(s), then the argument is invalid.