It is possible that your monitor does not support HDMI audio. I think this is a pretty new capability (could be wrong about that). In any case, as vincent5487 said, check to see that your sound source on the monitor is set to use the HDMI stream, if you can do so.
rubberman
Posting Maven
2,572 posts since Mar 2010
Reputation Points: 365
Solved Threads: 306
Skill Endorsements: 52