A good overview of the issues surrounding the introduction of voice, is in the article ‘Vox Lindeni‘, by Prokofy Neva at the Second Life Herald, which includes a mention and link to this article:

Not Yet you Fools! By Richard A.Bartle
Online at GameGirlAdvance, written in 2003, about the introduction of voice into multiplayer online games.
Sample quote: ‘If you introduce reality into a virtual world, it’s no longer a virtual world: it’s just an adjunct to the real world. It ceases to be a place, and reverts to being a medium’.