Copied from http://www.swtor.com)

TOR will be the first fully voiced MMO – Every playable character (male and female) and every non-playable character in the game will be brought to life by voice actors - resulting in several thousand speaking roles and hundreds of thousands of recorded lines of dialogue! This gives players a compelling and immersive story telling experience.

My concern is that the guilds who've advertised in the Outpost and are feasible for my play style all either require or strongly encourage the use of Ventrillo. I myself have never played a voiced MMO, nor have I used Vent, but have used Teamspeak in the past and, if they work the same way, then I can't imagine having to concentrate on hearing what I'm being told by an NPC quest-giver AND listening to what could be important instructions in Vent. Not only that, but when in a highly populated zone with many merchants and quest-givers, there could be tons of background voices from other people talking to other NPCs at the same time. Does anyone have any experience with this? I foresee the duelling voices in my head getting very annoying very quickly. Or maybe I'm making it out to be more than what it really is.

My guess is that they'll have some VO pause script when teamchat is in use. If all else fails, then I'm sure that the dialogues will be scripted in text in tandem with voice, like most other RPGs.

Like Pastrami said they'll almost definitely have subtitles, and I don't think you'll be able to hear other people's conversations because that'd be distracting and could spoil the story for you if you heard a guy talking about what happens 3 quests down the chain.

And what kind of guild makes you use Vent when you're questing?

I don't think it will be that hard. If you're in the middle of a raid, for instance, and the raidleader is giving out instructions while in-game dialogue is going on, then he's a bad raidleader. If you're just questing on your own, turn down your headphones when a dialogue scene comes up. I also don't think you will be able to overhear someone elses quest dialogue, think of it like a mini-instance where you get a Mass Effect style cutscene but no one around hears it or can interrupt it or interfere with your quest in any way. Many games have had voice overs for NPCs and it's never been a problem, if you haven't played Mass Effect, I'd recommend checking it out because I think alot of this will work similar to that game.

And I hope they don't have subtitles, keep conversation moving fluidly. Maybe if you stop responding it can show a subtitle of the last line that was spoken like Mass Effect, but in general, no subtitles.

I would guess that they would probably have an option to turn sound effects and the like down or off. I know when I still played WoW I usually turned the game sound effects off when I was using ventrillo or teamspeak in a raid or otherwise.

I doubt there will be much in-game voicing at points where members of a raid would typically be speaking.

I could see bosses doing Macbeth-esque monologues, this is a space opera setting, you know. o_Q

Hah true, well that is when you issue the "STFU!" raid warning. :p

I wouldn't say I'm concerned, but with MMO's it takes the customization to a level that the Character (Unless you RP) is essentially you... and it's not like the Voice actors they employ will have a Middle England accent haha. I didn't like it in Mass Effect but then My name isn't Adam Shepard and I'm not an American so I lived with it.
For the sake of Argument I would probably Mute any voice acting from my Character and stick to Subtitles.