I found recently that I didn’t hear any audio from HDMI. Googling revealed this: http://www.raspberrypi.org/phpBB3/viewtopic.php?t=5062
Following those tips, I ssh’ed into the rpi and did this:
sudo nano /boot/config.txt
And added this at the end:
Then hit control-o to write out the file, control-x to exit, and reboot.
I remember having done this before and I realized my previous change was probably overwritten by a recent auto-update. So just in case, I disabled the auto-update feature from “Programs->Raspbmc Settings”.