I'm in the reverse situation... i see the picture very well.. but no sound and like you went thru the whole routine of check the ports n plugs... tighten them as much as you can... switch it on and off numerous times to reset the blasted thing... but nope.. no effect watsoever.. and finally the comcast tells me that theres likely a problem with the box wiring so take it back to the comcast office for a swap
drat! I've spent atleast a couple of hours every week for some problem or the other with my comcast connection...
In some cases pressing the LANG button on the Comcast remote takes care of no sound. This only works for the small DTAs.
If you have a larger box you need to check the menu settings for some sort of SAP setting. Even double check your tvs audio settings for this using the tv remote and tv menu.
If you still have problems please post back with the model of the box you are using.