Personally, I find that either way, voiced or silent, can work or flop miserably.
In the defense of silent protagonists, I call upon Caim from Drakengard, and The Hero from Dragon Quest 8 (Who will be referred to as Eight, as the promotional images called him).
Two characters, both from JRPGs, on opposing ends of the JRPG Idealism/Cynicism spectrum, with their own stories to experience. Now, a silent protagonist in a JRPG is a common trope in of itself, but I think these two really work well for it, in terms of why they are this way, and how they express themselves otherwise.
Caim, who was conceived as a dark rebuttal and parody of the typical JRPG protagonist (young, silent, idealistic, etc), was 24 (according to the ingame booklet), a bloodthirsty soldier in a war with the motivations of revenge against the opposing side for varying reasons (mainly, the threatening/death of loved ones caused by said side), and generally an unpleasant fellow. However, his is our protagonist, so we have to follow him around. In the briefest of moments in the beginning of the game, Caim is voiced for a short time, however, due to an instory reason within the game, the power of communication is stripped from him.
That reason being, Caim had preformed a magical and soulbinding contract with a dragon, with which the voice was his price. Alongside losing his voice, Caim was able to save his life (he was struck with a mortal wound moments prior) and gain immense power via working with the dragon. This ritual, known as a pact, is acknowledged multiple times in universe by other such pact makers, and are shown the varying ways a pact can take something from a mortal. A man loses his sight to gain power from a faerie, a woman her womb to team up with a pair of elemental spirits, a 6 year old boy unable to age in order to work with an enormous and powerful golem. These other examples help justify Caim's new mute state, as well as force him to communicate to the audience (and other characters, naturally) how he thinks and feels about his experiences. And while the games animations aren't the best, I think that they do a damn good job showing how Caim felt about a given situation.
Now, on the opposing end of the spectrum, we have Eight. Probably in his early 20's, and always silent. Like the Gordon Freeman mention before, I think Eight is supposed to be a projection, as you can name him, and the game actually does a pretty good job to skirt around the fact that you named him, with an in universe nickname, or dialogue that doesn't require his name to be used at all in reference to him. However, unlike most blank slate protagonists, Eight does in fact have a bit of a personality to him, even if it's not as well rounded as the other cast of characters. He's kind, altruistic, and a bit on the snarky side (just watch him if you pick no on any "But thou must" choices). Furthermore, he shows a willingness to forgive others for past deeds, but not to the point of stupidity. And I think it is there he really shines, and deserves his mantle as "The Hero", especially in this scene here:
Now, if you didn't watch that, I'll try to summarize it. Essentially, one of your travelling companions left his filthy hometown in hopes of escaping the life of crime that is expected of residents there, only to find he began stealing again to survive. Enter Eight, who is a retainer of his king, their cart and horse; who are trying to cross a bridge that the thief character had taken and demanded a toll from. Eight defends his king, the cart and the horse, and gets himself, said king and their possessions across safely. However, in the scuffle on the bridge, the stability of said bridge is compromised, and the thief falls, barely holding onto a rope for dear life. Now, while the king says (grossly paraphrased) "Ha, Karma bitch!", Eight turns around and goes to help out the thief, even when his KING, the man he is serving and protecting, as well as just saved from an attack, tells him "What are you doing? This thief was about to kill us!" And yet, Eight saves him anyway. Furthermore, his kindness to the thief turns out for the better, as the thief will become one of your best assets to your team in game, as well as the most reliable of Eight's comrades. But Eight didn't know that. He probably didn't think that the thief will be of any help to him. His own King tells him not to, and says that it's the thief's just deserts, but Eight did it anyway.
And it's because of that, I don't find his silence irritating. In fact, his movements and actions speak far more than words ever could for him; without coming off as hollow or puppetlike.
And it's for characters like that, I can endure and sometimes enjoy a silent protagonist.
EDIT: Wow, I Really should have read the other posts before my own. Oh well... But looking back at this, I realize I may be coming off as favoring one over the other, but in my post above, I was merely trying to show how a silent protagonist can work, and work well. That does not mean I don't care for voiced protagonists. In fact, I love protagonists with their own story and development, that's why I love RPGS!