I was not able to get sound out of hdmi running radeon or radeonhd. I didn't see vesa as a module (it must be automatically part of kernel as there were no kernel options regarding it). I made xorg.conf use the vesa and X came up. No sound over hdmi in that either.
In all three cases I can no longer get sound out of front headphones but it is working over optical cable well in all cases.
So it appears fglrx must be loaded to get sound over hdmi.
I thought it might have been solely due to the kernel. When bulding this one in make xconfig
----M for Intel HD Audio
------X for Build Realtek HD-audio codec support
------X for Build ATI HDMI HD-audio codec support
------X for Enable generic HD-audio codec parser
I chose these, realtek because mboard says has realtek azalia; ATI hdmi for obvious reasons, and generic just in case needed.
I'm guessing now you need these AND fglrx for sound out of hdmi.