The volume is very low & I can't find any way of increasing it; I don't want to add separate speakers as I have my phone linked to a headset via "internetcalls.com",in which the volume to the earphones is independently cotrolled; but this does not allow one to adequately hear the monitor's speakers & as my wife & I often talk to family, we can't both hear the sender's voices.

If you are referring to a computer with your microphone & headset attached
click start control panel sounds and audio devices select advanced there might be a tick in the mute box untick it or select audio from the microphone drop down list make sure you have the right one selected and in the volume box make sure the microphone box has a tick in it or select the voice tab and make sure that you have the right media selected also select volume make sure the box for microphone has a tick in it
click start control panel sounds and audio devices select advanced there might be a tick in the mute box untick it
click start control panel on the left select hardware and sound then select manage audio devices then recording you should see a microphone right click select enable windows 7
hope this helps