your computer presentation includes sound and you did not connect
it to the projector's HDMI port,
you can still play sound through the projector's speaker system.
Just connect an optional 3.5 mm stereo mini-jack audio cable
as described here.
Connect the audio cable to your laptop's
headphone or audio-out jack, or your desktop's speaker or audio-out
Connect the other end to the Audio port that corresponds to the
Computer port you are
Note: If you
are projecting video through the projector's DVI-D port, connect the end of your audio
cable to the Audio3 port.