Right-click on the screen of the xlite then select options. Once there, click on devices on the left ( Icon looks like a headset) and click on the "microphone device" drop-down within the "Headset" section. Select your microphone and hit save.

If your microphone is already selected and active but still does not work. Make sure there is no mute feature on it that may be turned on.

Try a new headset, if the new one works, it is likely that the one that did not work is broken.