Receiving distorted sound through HDTV speakers.

Recently realized that my sound card does not support sound through HDMI cable. I connected a S/PDIF cable to the output port on the PC and the other end to RCA inputs to HDTV. I have an HDMI to PC and other end to the HDTV. I have no issues with picture quality. The sound on the other hand is poor. (distorted) I am not looking for HD sound just something that can be listened too. The specs and tech support say this will work. I will get sound through ether the amp or the TV speakers. Not sure what I am missing or what to do to correct this issue. Am I missing drivers? Is this card defective? Unfortunately tech support is no support when dealing with Gateway. Contacted NVIDIA and they reverted me back to gateway. They will not support products that are for OEM at least this is my understanding. Is there something I can do to correct this problem? I can really use some input.
Thanks.

My specs are:
OS Name Microsoft® Windows Vista™ Home Premium
System Manufacturer Gateway
System Model LX6810-01
System Type x64-based PC
Processor Intel(R) Core(TM)2 Quad CPU Q8200 @ 2.33GHz, 2336 MHz, 4 Core(s), 4 Logical
Installed Physical Memory (RAM) 8.00 GB
Total Physical Memory 4.00 GB
Available Physical Memory 5.73 GB
Total Virtual Memory 16.0 GB
Available Virtual Memory 13.7 GB
Graphic card NVIDIA geforce Gt 120