A sound argument comes as one where the premises are true, and the conclusion is true. For example:
makes for a sound argument as does:
Case 3 doesn't hold, because neither does it hold that Teller is ten feet tall, nor does it hold that he hasn't taught logic.
One reason the notion of a valid argument becomes important, because we might end up studying or working with some system where we don't know whether the premises hold true, or if they hold false (but we can tell that one of the two must hold... or we know what the possibilities are). For instance, suppose that we know we have some particular binary operation % given to us. We're given the following argument
The argument holds as valid no matter what "%" indicates, because of the structure involved here. The premises could hold true... say if "%" indicates addition. But, the premises could hold false also... say if "%" indicates multiplication. Either way though, the argument holds valid by its form. Consequently, we can tell something about "%" and know that certain consequences will follow even if we can't figure out the truth of the premises.
As perhaps a better example to illustrate why validity can be important, one can argue:
If a set of axioms A is independent, then there does not exist a model of the axioms which has some property P.
There does exist a model of the axioms which has some property P.
Therefore, the set of axiom A is not independent.
This argument holds as valid. But we can't tell a priori whether or not for a given set of axioms there exists a model of the axioms which has some property P. But, the argument holds as valid nonetheless and thus enables a sound argument, in a particular case, if we can find some instance where the premises hold true.