Cable TV does support Dolby Digital surround sound IF the channel has it. You can select that option in the cable box set up menu.
If the TV audio menu won't allow you set the TV to pass the audio in 5.1 then your option would be to use a separate digital audio out from the cable box to the LG. If it doesn't have one you can use an HDMI switch with audio extraction to derive a separate audio feed for the LG. Set the LG surround mode to Dolby if you can. This will give you Dolby Digital if it's present and Dolby Prologic2 when the program is in 2.0. Try any other modes the LG has to see which ones you like.