There should be a setting in the TV menu that lets you output sound from RCA.
On my Panasonic plasma it's like this, since sound is integrated into HDMI and coaxial by default. So you have to tell it to output to RCA, since usually it just sends sound out through the same cable as the video.

HP Media Center machine
MOBO: ASUS OEM board
CPU: 2.6 GHz Socket AM2 Athlon X2 5000+
RAM: 2 GB Qimonda PC2-4200 DDR2 RAM
GFX: Sapphire Radeon HD 4850
HDD: Seagate Barracuda 320 GB SATA 7200 RPM