mac mini hdmi sound issue

I purchased a mac mini. One of the new ones with hdmi. When I plugged it in everything seemed fine. Then I noticed the sound was real loud. So I went into the volume control area. I then discovered the hdmi volume setting is locked. I can't turn it up and down. I have my PC plugged in the same exact way and can control the volume within windows. Why does the mac not let me do this. Also is there something I can do to fix the problem.