If this was FS9, I would probably recompile the voicepack file, check the settings to see whether the ATC had been turned off in sounds and do that kind of troubleshooting. I know the voicepack software changes for FSX, so I don't know whether there is a difference in file structure and that the system might have different failure modes. I'm surprised that this problem hasn't come up in FSX.
It may just be some facepalm event, like you enabled headphones and then unplugged them and forgot to re-enable speakers. It's more likely to be operator error than some random system glitch.
