Make sure you have the latest version of Live Meeting:
The fix that worked for me was the change the default Playback Device:
- Right click the Audio/Speaker Icon in the Windows Task Bar and select Playback devices
- Right click a different playback device and select Set as Default Device
If that doesn't work go back into Live Meeting and play around with the Audio options:
- In Live Meeting select the Voice & Video menu
- Click Options -> Setup Audio and Video
- Try changing the Speaker you want to use.