Hi,
Not sure I understand correctly but if you are definitely getting audio through from the PC output, that you can hear with headphones or loudspeakers, then I suspect that the problem lies with your settings on the TV.
On my LG flat screen, if I want to hear audio from an external source (PC, IPod etc.,) I have to alter the audio settings on the TV itself as it doesn't auto detect the signal. Check your TV user manual to make sure you have it set-up correctly for audio from an external source.
Hope this helps