HDMI not recognized for Sound Output

I have an early 2011 MacBook Pro running Yosemite 10.10.3. I've connected my laptop to an ASUS VE258Q screen through a Mini DisplayPort to HDMI cable. The video works perfectly. However, the sound is coming from my internal speakers instead of the ASUS speakers. I went to Sound on System Preferences, and under "Output" there is no HDMI/ASUS device for sound output being recognized. Is there a way to ADD the HDMI/ASUS screen for sound output? I've also gone to applications --> utilities --> Audio MIDI Setup and there is no HDMI/ASUS being recognized there either. I would like to have sound through the ASUS screen. Please help!