I would not recommend Bitstreaming.
The Lexicon is not going to show up as your Audio Device since you are using HDMI (your Audio Device is your HDMI port). Right click on My Computer -> Properties -> Device Manager -> Display adapters. You should see your HDMI port there--either your add on video card or what is directly on your motherboard.
In MC, go to Tools -> Options -> Audio -> Audio Device and select your display adapter with the [Direct Sound] option. You should not have to reboot, but you may want to close Media Server (not just Media Center) and then re-open Media Center. Do you get sound? If not, post again.
If you do, start working your way up. Choose the same display adapter except with the [WASAPI] option this time. Do you still get sound? If yes, you're done.
If no this time, check Tools -> Options -> Audio -> Audio Device -> Device settings -> Device and check "Disable event style." If yes, you're done. If no, just go back to Direct Sound--that's the best you can do.
The best possible outcome in order is WASAPI Event Style > WASAPI Event Style Disabled > Direct Sound. Work your way up though first trying to get just sound through Direct Sound.
Note: Do not create a Zone for each possible Audio Device selection; you will not use them all (you just need the best one that works). Stay in one Zone and trouble shoot from there.