It's not just Neverwinter... I'm getting the same "unable to authenticate" with the Star Trek Online & Champions Online launchers as well. ARC, the ARCgames site and Neverwinter Gateway all log in fine ... it's just the actual games that are failing to authenticate. It's been like this for me for the last 6+hrs. :(
Or ... (more awkwardly) ... set the in-game voice-chat to Push-To-Talk mode rather than open mic. ... ... Assuming you're both playing in the same room, physically that is.