The until now beta tested voice feature of the First Look client has gone into the main client. Thus it is enabled everywhere and now mainstream.
The use of voice is always going to be optional, and estate owners can disable it, if wanted, but it's now there and without any extra charge at all for the sim owners, as it seems.
While some couldn't await the arrival of this feature into the main client any longer, others are still sceptical about it and/or don't want to use it at all. Consider it working under Mac OS X and Windows, but not under Linux yet.
It's simply not working under Linux because the proprietary binary that's needed to connect to the servers of Vivox is still non existant, meaning what you've got under Linux there is an empty shell so far.