Wise men talk because they have something to say; fools, because they would like to say something. As empty vessels make the loudest sounds, so they that have the least wit are the greatest babblers.

Often attributed to Plato, it cannot be found completely in any of his writings (see this). The quote is attributed to Plato in A Dictionary of Thoughts: Being a Cyclopedia of Laconic Quotations from the Best Authors of the World, Both Ancient and Modern (page 560) by Tryon Edwards.

In the Symposium (see above) Plato likens empty vessels to fools for their lack of content, but says nothing of their sound, because clay vessels and not wooden vessels were commonly used in the day of Plato, and the popular common saying about empty barrels would have become common knowledge only once the wooden barrel later had become a standard vessel type. | In the Laches (see above) Plato includes a criticism of one who has "no meaning" and uses "empty words" to "dodge" "in the hope of concealing his own perplexity". A conflation of these two quotations may be the origin of the spurious quotation given above.