Media center no sound with TV and Can't change channels

I have a SR1750NX Compaq Presario media center with Media center 2002 serice pk 2. I added the Hauppauge pvr-250MCE. I use a digital coax cable from my computer to my stereo receiver. I can hear windows sounds, music, ect. But when I try to watch live tv or record video through media center. There is no sound. I have switched the rca audio cables with ones that I know are working but it didn't help. This is not a new computer but I finally got a capture card and remote to use with it. Another problem is the remote will not change the channel.