Your speaker volume is too low and you might not hear other people on the call. Try increasing the speaker volume from the Control Panel or from the notification area.
If other people on the call cannot hear you, try increasing your microphone volume as follows:
In the lower-left corner of the Lync 2010 main window, click the audio device menu , and then click Audio Device Settings.
Under Customize your device, drag the slider bar next to Microphone to an appropriate level.