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
BNC port you are