Weak audio from speakers when monitor is connected to HD set top


I have a BenQ VZ2250 monitor with HDMI feature. I have connected my monitor to a HD Set Top Box thru a HDMI cable and also connected my 2 speaker 3.5 mm male jacks to set top box female ports (Left & Right Audio ports) but, the sound coming out of the speakers is very low but the video is perfect.
I tried to change the "playback" tab setting but there is no HDMI written there. There are only 2 options, one is Speakers and other one is Realtek HD audio device.
My OS is Win7. Kindly help me out.