Use the Volume buttons to adjust the volume, or adjust the Volume setting in the Settings menu.
Make sure you are using a "no resistance" audio cable.
Make sure A/V Mute is not enabled. (Press the A/V Mute button to disable it.)
If your presentation includes computer audio, make sure the volume control on your computer is not turned all the way down or muted. In Windows, you may be able to check this by clicking the volume control in your Windows system tray (lower right corner of the screen). On a Mac, you may be able to adjust the volume from your menu bar (top right corner of the screen) or via the volume control function keys.
If you are using a microphone connected to the projector's Mic port, make sure the cable is connected securely and the Mic Input Volume on the Settings menu is adjusted correctly. See Customizing Projector Features.
Also, make sure you are using a dynamic microphone. Condenser microphones will not work.