I too am having the exact same problem. I have a VAIO RZ504, running XP SP2.
I have recently installed a USB external sound card (Sound Blaster 5.1 X-Fi), but have since lost all sound from live inputs via the Giga Pocket TV card. However, any previously recorded files still work. Also, if I set my system back to the onboard sound card, I can record with sound, and then playback using my USB sound card - It just won't accept sound via a live feed. The volume is constantly at zero, and cannot be moved
Here is a comparison of the two sound card's volume controls:
As you can see, the new sound card is lacking the crucial 'Volume control' setting which seems to be the one that the GigaPocket software is dependant on. The volume controls on my VAIO remote also no longer work