Intermittent voice chat? on MacBook Pro

I'm Support for students that are using MacBook Pros and need to use voice chat in class that will be in SL.

I've successfully used Voice on the iMac I use for support with no problems. I've confirmed their system preferences, and when we go into SL they will 'sometimes' have voice chat sometimes not. We've gone over their settings in SL, but when they go into Voice Chat and Device Settings it just sits and says 'Waiting'. This is with his headset plugged in before he logs in and with either default settings or set to the headset mic input.

Any ideas?

Further info...

Upon researching it more I was able to duplicate this on my iMac. It appears as if there's some sort of bug between the 'preferences' option from the login page and the preferences option within SL. I would log out with it enabled and when I logged back in it couldn't detect my device. I had to enable it within SL and then it could detect it and then disable it before logging.

Any thoughts?