In general, you do not know whether the premise is true or not. Still, we would like to say that something is wrong with an argument, if its premises are wrong. Note that
All men are immortal.
Socrates is a man.
Therefore, Socrates is immortal.
is a valid argument. But presumably, it is not sound.
When calling an argument "sound", you defer the questions of knowledge and certainty. But usually, an argument should not only have premises that happen to be true, but premises that we find plausible (in which case, you might call it an persuasive argument).
You can have arguments that are valid, sound, persuasive; but usually not absolutley certain.