I have been having problems with my sound fading out after I installed a hauppauge PVR 250 tv tuner/capture card in a PCI slot. I have onboard sound with my Asus P4C800 Deluxe motherboard and if I unplug the sound cable and plug it back in it works again. Any suggestions would be appreciated.

P4 3.0 Ghz
1 GB PC3700 Kingston RAM
XFX FX5200 128MB video card
Asus P4C800 Deluxe Motherboard

assuming that the TV card is linked to the Sound card via a CD-Rom styled analog cable, i would say that is some kind of eletrical fault on one of the cards, and if the onboard was working before then i'd be inclined to remove the TV card to check if that actually causes it.
In the past i have had problems with sound cards sharing IRQ's with other cards seemingly ok, but causing a fault somewhere, you could check to see if the sound card is sharing resources with the TV card.
maybe by coincedence your speakers may have a fault, try a different pair maybe. as always make sure you have the most up to date drivers for all hardware, and a god performance tip is to install the latest Directx after any driver upgrades, which includes addition of new hardware.

With your description it sounds like a phisical prob, you should check if your connections aren't coming loose, and the cable should remain tight on both ends. Also try and see if a diferrent cable helps.

Are you plugging the sound for the tuner card into the computer for sound or directly into an external powered speaker system? I have a HP tv deal as well and I initially had the sound plugged into the sound card. Sound was terrible. I then plugged it into the secondary input on the subwoofer satellite speaker bos and sound worked perfect. AJW86 is prob on the right track with the cable TS solutions.