Silence is the virtue of fools.
Francis Bacon, De Dignitate et Augmentis Scientiarum, Antiteta 31, 1623
Silence is not always tact and it is tact that is golden, not silence.
Samuel Butler, The Note-Books of Samuel Butler, 1912
The cruelest lies are often told in silence.
Robert Louis Stevenson, Virginibus Puerisque, IV, 1881
Silence is consent.
On the other:
I have often regretted my speech, never my silence.
Publilius Syrus, Moral Sayings, no. 1,079, 1st century BC
When you have nothing to say, say nothing.
Charles Caleb Colton, Lacon: or, Many Things in Few Words, I, no. 183, 1825
Do not the most moving moments of ours lives find us all without words?
Marcel Marceau, 1958
All of the above from Words on Words by David and Hilary Crystal, University of Chicago Press, 2000.
And then there's the happy marriage of silence and speech:
It's said that when John (Edward in some retellings) Kennedy was first campaigning, someone in the crowd yelled out, ‘You haven’t worked a day in your life,’; in the silence that followed, someone piped up, ‘You haven’t missed anything.’
(Thanks Billy for that one.)