FWIW, I would make your narrations outside of PowerPoint,and then insert them into your presentation. the sound quality in my estimation is far better. I have no idea why your particular microphone does not work within PowerPoint

 I bought a USB microphone (Plantronics 510 USB) to make lecture narrations in
 Powerpoint.  The microphone works in Windows Sound Recorder fine.  

 I open a Powerpoint file, select Slide Show - Record Narration to get the
 dialog box - so far so good.  When I however press the 'Set Microphone Level'
 button I get the message: "PowerPoint is not receiving sound from the
 microphone. Try one of the following and then try to set the microphone
 recording level again. *In Control Panel (Windows Start menu, Settings
 submenu), double-click Multimedia. On the Audio tab, check the Recording
 settings. *On the Language Bar, click an the microphone button to turn off
 Office speech recognition."  In don't see a MultiMedia menu in Control Panel.
  (I'm running Windows XP.), but I have turned off speech recognition with no
 benefit.  

 I called the company but they had no idea why it wouldn't work in Powerpoint
 if it was working in Sound Recorder.  Any idea what's wrong and if there are
 any work-arounds?

 Thanks,
 Cory

Try this:

Start, Control Panel, Sounds and Audio Devices Audio tab (NOT Voice tab, odd
though it may sound) Select the USB headset for both the default sound playback
and recording devices

Click Apply

-----------------------------------------
Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
================================================