External monitor on HDMI but no sound
From what I can tell from reading the Specifications Page, there are no speakers built into  this particular monitor.  There are however, two different audio outputs:  HDMI 1.3 and 3.5mm headphone/speaker jack



The easiest way to test this out is just get a cheap set of speakers or a headset with the 3.5mm jack and plug them in.  If it works, you know all is well with your setup - you just need speakers.