My sisters has a problem whereby her voice waivers a lot when shes on the phone or doing group speaking. Her issue was related to nerves, which can effect people in different ways. Not being an expert, but it does sound as though you are not confident when perhaps in other scenarios you would be.
One issue you could have is that you don't have a problem playing to a large amount of people whom you may know and dont know equally. However when you are forced to speak to a smaller group of people who you don't know, your confidence leaves you.
Think you need to try and force yourself to think that nobody is in any position to judge anything you say, and that if you just get up and do it you might be better off than worrying about it.
For futher assistance I guess you could try calming tablets and suchlike, they do work apparently. Its all about your will to succeed I guess, even if you work wasn't good, if you don't act confidently, you wont come accross as confident and unfortunately confidence is one thing potential managers could benefit from.