Not sure.
Try one or all of the following:
Go to TV Options, Advanced Properties page.
1. Double click "Audio Capture Filter...". In the resulting dialog window, select Line-In (assuming that is where you connect your TV card to your sound card). Make sure the "Enable" checkbox is checked. Click OK.
2. Double click "Video Crossbar..." or "Second Crossbar ...", in the resulting dialog window, select from "Output" drop-down list "Audio Decoder Out". Then make sure "Audio Tuner In" is selected in the "Input" drop-down list. Click OK.
3. Go to "Audio" page, experiment with turing "Enable Audio Mode Selection" on and off.