I don't think you can, not via HDMI. HDMI is a great interface that was invented for home theater audio and video equipment. Monitor makers (who typically happen to also be big screen TV makers) do not want to support two digital video connections that carry the exact same video component to their monitors, so HDMI is the standard that is winning out over DVI (which carries no audio). But the truth is, HDMI does NOT port to computer systems very well because most computer users use self-amplified computer speakers instead of a HT receiver that then powers home theater speakers. Monitors with integrated speakers are still emerging but due to the size of the speakers and limited built-in amplification circuitry, monitor/TV speaker simply cannot deliver "high-fidelity" sound and are not suitable for most movie watchers. The audio from such monitors is tinny, no bass, and not surround.
In other words, HDMI is not the best interface for audio for most computer users - and definitely not in multi-monitor setups.
What you need to do is NOT run audio through HDMI, but rather use the discreet audio outputs of your computer's sound device and split the audio to both monitors/TV (or perhaps, left audio to the left speaker on the left monitor and right audio to the right speaker on the right monitor).
Some monitors have "pass-through" support to where you can take the audio out of monitor 1 and feed it to monitor two, but there may be an very slight, but noticeable delay, or added "noise" and that is not good fidelity.
There really is no good solution here, except to use separate speakers - which is what I personally recommend because I have yet to hear a monitor, or big screen TV that had anything near high-fidelity speakers or sound.