If I'm reading this forum thread correctly, and please correct me if I am wrong - even newly purchased 360s like mine (250GB holiday bundle w/ fable3 and Halo ODST) you cannot hook up both HDMI and optical audio outputs and get both to work (as in my case). I get stereo audio through my TV via the HDMI connection, but no evidence of any audio being pushed out the optical cable.
In order to achieve this, I have to buy yet another cable from microsoft or monoprice to achieve this function - even though both HDMI and optical outputs are provided on the xbox itself. This is ludicrous to me, my PS3 allows this function to occur and even gives me the ability to change the video/audio options - per connection type - in the setup menus.
If Sony can do it, why can't Microsoft? I can't be that difficult of a thing to program into the hardware or the menus. Even my Tivo will allow me to do this without much trouble.
Yes, the easiest thing to do would be to replace my aging 15 year old receiver with a shiny new model that supports HDMI switching and pass through - but I guess to fix the problem with a $3 cable from monoprice (though they will charge me $5 to ship the darn thing) this will get me through.