Sorry. Should have asked for the Blu-Ray player model as well.
Since the LG Sound bar only has two types of inputs, you should be looking at the Blu-Ray manual for audio outputs (either stereo or optical) and using the appropriate input. Now, depending on the blu-ray player model, it might not be able to separate the audio output from the video output (I'm assuming HDMI here).
I'm also assuming we're talking about the UHappy U80 projector (please correct me if I'm wrong in either case). If you can't use a separate audio output on the blu-ray player, it's likely you would need to run a 3.5mm stereo cable from the headphone output of the projector back to the stereo input of the sound bar.
-Wolf sends