When I fist tried to use voice, it always told me that the hostname could not be resolved. I searched a bit around, then i found the problem. The Vivox client tries to use IPv6 if available. In most cases the newer Linux Kernels do have IPv6 compiled in as a module, so there is a good chance that IPv6 is activated.
So if you have general voice problems and the white dot does not appear, take a look in the debugging console from SL and look out for some Voice-Errors while disable and reenable voice two or three times. The hostname error does not show everytime becaus the client tends to go to an unknown status when the hostname could not be resolved.
To solve that problem, i recomplied the kernel without IPv6 support. But maybe you have another good idea how to disable IPv6 in your linux distribution.