I have similar problems with the sound on Raspbmc. If i put the headphones the sound comes.
As I have read here I have to buy a cable 3.5mm Audio Jack (male) to RCA Audio Converter and connect of course to the Audio output of my raspberry pi.
My TV has just DVI and Minicinch.
My setup is the following :
HDMI to DVI cable from raspberry to the TV
Minicinch to cinch (red/white) from the TV.
Is it just enough to connect the RCA Converter to my Minicinch to cinch or have I to run some commands in the terminal ?? The Raspbmc has no terminal. I didn't find one
Thankx for your help