We have now completed our voice maintenance and the inworld voice service has now been restored. If you are in a voice-enabled region and your Talk button is still grayed out, please go to Edit -> Preferences -> Voice Chat, de-select Enable Voice Chat, select Apply, and then re-select Enable Voice Chat and click OK.
As previously announced, we have begun a scheduled maintenance outage for inworld voice. We expect the voice service to be back up at around 3am PST (11am GMT). I will post again when the maintenance is complete.