Before there was Second Life voice there was Skype-- a service that allowed free voice and webcam calls between (or among, as there can be multi-way calls) internet users. Sweetie and I began using it in late 2006 or 2007.
It was strange to hear and see one another after a month or so of typing and watching our avatars kiss on the Devotion poseball, but we quickly got used to speaking to one another and soon Skype became our primary method of communication.
Almost always when we're in world together we're in Skype, keeping up a running conversation that ranges from the sublime to the ridiculous-- usually the latter. It was in Skype we planned the sim we were buying, in Skype we designed our robot sanatorium. We're in Skype while we're dancing, in Skype when watching video, in Skype when we're shopping.
When Second Life voice was introduced in the summer of 2007 we gave it a try, but in those early days we found it glitchy and were soon back in Skype.
Nowadays voice works pretty well on the grid. I usually have it enabled, but I'm rarely interested in saying anything. Most of what I hear-- especially at the infohubs/welcome areas-- is inane and I've just no incentive to speak. If I want to say something, I prefer to say it in chat.
At most of the places Sweetie and I go, even where there are lots of avatars-- for instance, at concerts-- people use rarely use voice chat. The few who do are often profoundly annoying to everyone else.
And yet voice statistics for voice use are high (or so claims Linden Lab, who apparently track every minute every avatar has voice enabled). I'm sure most of the actual use is between couples and small groups of friends, either in public through the private voice channel or at their homes in open voice.
Time has shown the naysayers who predicted the death of Second Life at the hands of hordes of screaming avatars to be wrong. Wrong, too, were those who believed voice would significantly change the grid. What has actually happened, I believe, is something no one predicted (or at least I don't remember it ever having been mentioned): people use voice to talk privately among themselves.