I just bought that same television and had the same issue. Go to the RCA website to install new audio software:
http://tv.rca.com/en-us/softwareupgrade.html
Install the program named:
TTE05-3.13.08.08_522US_Software.upg
Hope that helps.
I bought this TV the other day:
http://tv.rca.com/en-us/modeldetail.html?MN=L42FHD37
It's hooked up to my DirecTV HDDVR via HDMI.
I've tested all three HDMI ports. If I turn the TV on, I lose sound. To fix the problem I either have to unplug/replug the HDMI cable OR I have to cycle through all of the TV inputs. I have no idea how to fix this.
Any suggestions?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic