I just added another monitor to my desktop setup using an HDMI cable. This particular monitor has on board speakers but when I plug in headphones in to my tower it will kill the sound on whatever I'm watching (no sound in headphones) AND the sound remains off even after I unplug the headphones. If I exit out of the browser and try again the sound will work with no problems.
Previously I had been using the audio out jack in the rear of the computer and running it to some kind of iphone charger/speakers deal that I had no other use for (I didn't own a iphone). Plugging/unplugging headphones worked with no issues.