Hi hogo, by looking at the specs of your PT470:
Input / Output Digital Input 1 (Optical)
HDMI Out 1
Phone Out 1
USB Terminal 1 (for MP3, WMA, JPEG, MPEG4, DivX®)
Audio Input_ 1 (AUX)
your only option for a digital input is with optical/toslink.
1. Your TV does not have optical in/out.
2. No HDMI input on the PT470 only optical.
3. By wanting to use HDMI I assume you do not want to use SCART, which I am unfamiliar with but understand it is analog.
About the only way I can think of getting audio through the PT470 is to use the optical out on the xbox cable to your Cinema on an input of your choosing (if assignable) and HDMI to one of the other HDMI inputs for the tv, then mute the TV and crank the Cinema. This would require you to crack open the plastic casing of the connecter at the xbox end of the cable (and left off) because the standard connector and the HDMI cable will not fit in at the same time.