If your presentation includes computer audio, make sure the volume control on your computer is not turned all the way down or muted. Make sure the audio cable is securely connected to both the projector and your computer. See Playing Sound Through the Projector.
The wrong source may be selected. Press the Source Search button on the projector or the Search button or one of the source buttons on the remote control.
Make sure you are using a “no resistance” audio cable.
If you are using a microphone connected to the projector’s Mic port, make sure the cable is connected correctly and the Mic Input Volume option on the Settings menu is adjusted correctly. See Customizing Projector Features.