A Kiwi friend of mine who has lived in England for many years discovered a speaking problem when she became a professional stage manager in London's West End.  Here's how she described it:

"I can clearly remember the first time that [as stage manager] I had to make announcements to the front of house from backstage through the [speaker] system.  Most stage managers do this to get the audience to take their seats prior to the start of a performance if there are no warning bells. I was so petrified at the idea of hearing my voice, with its very strong New Zealand accent, that I asked the resident technical manager to do it for me."  What bothered her, she realized, was the sound and feeling of her own voice booming out there amplified.

She's not alone.  Hearing yourself through a microphone can make you feel like you're speaking from a great distance, far away from your audience. That booming sound can be terrifying because you feel like you absolutely mustn't make a mistake.  Every tiny misstatement will be multiplied a hundred times by the microphone's volume level.  At the least you'll feel distanced from your audience.  At the worst, you'll feel like a perfect idiot, forget everything you've ever learned about being a sexy public speaker and wish you could sink into the floor, never to be seen or heard again.  "It's most peculiar," the stage manager said, "being aware of the sound of your own voice remote from where you're actually standing while you're speaking."

Though they won't admit it, this is why some people hate to speak to large groups.  They're usually much happier and more confident in a smaller space where they can engage their listeners on a direct, intimate, eye-to-eye and voice-to-voice basis with no need for technical help.  However, the downside of limiting your speaking to small groups also means limiting your success.  The more people who hear you, the better known you become as an accomplished speaker, the greater will be your opportunities to attract more clients, sell more products, develop your management abilities, grow your reputation and become an admired expert in your field.  Now, isn't that a situation devoutly to be wished? 

           How did the stage manager conquer her fear? 

"After awhile," she reports, "I was so ashamed of being chicken that I plucked up the courage to do it myself.  I visualized the face of an actor I had recently worked with and liked (Derek Jacobi, though he doesn't know this, of course!) and pretended I was talking to him directly.  It worked!  I still use this technique when making audience announcements."

          Excellent solution, one of many.  The more you practice using a microphone, the easier it will be.  Recording your talk ahead of time and listening to it in the car or when you're exercising also helps you become accustomed to hearing the sound of your own voice outside your head, separated from you, so it's not such a shock hearing it through a sound system.  Make yourself comfortable hearing your own voice and your audience will enjoy hearing it, too.