Click down at the bottom right corner near the time and a little window will pop up

You will see a speaker on the left with a volume bar in the middle and an icon on the right. The icon on the right might show as an arrow or as headphones

Click the icon (arrow or headphones) that shows on the right

Under Input, make sure the Microphone (internal) choice is checked

To adjust the volume, move the dot (on the bar next to the microphone icon) to the left or the right