Up-- I'm having the exact same problem. This time, I'm connecting the Ps3 to my LG TV through hdmi, and the TV is connected to the LG HT906TA Home theatre through digital optical. The XMB sounds go off if I stop input for a few seconds, then sound comes back after I keep moving the XMB.

I didn't test a direct optical connection between, and I didn't use to have this problem on my older HT system with a direct optical connection.

p.s. HT906TA doesn't have Dolby Pro Logic. I was lured into buying it because I saw the Dolby digital logo alright, thought if it had dolby digital, it would have the preceding tech. I was wrong, and would like to return it, but I can't. (Also some websites report it as having Dolby prologic)