BrightLink 425Wi Interactive WXGA 3LCD Projector with Wall Mount

If you are projecting from a computer or video device using an HDMI or USB connection, you do not need to connect a separate audio cable to hear sound.

If you are projecting video using a Computer port, connect the projector to the video source using an optional stereo mini-jack audio cable.

If you are projecting video using the Video or S-Video port, connect the projector to the video source using an RCA audio cable.

Note: Older Macs may not support audio over HDMI and a separate audio cable must be connected to obtain sound.

Related references
Connecting to a Computer for Sound
Connecting to a Video Source for Sound