I have the same problem.
Found a work around for the sound disappearing with PowerVCR tv tuner. If and when the sound dissappears in PowerVCR TV tunning, close down PowerVCR and start ATI MMC and reset your sound settings/volumes if necessary. Close down MMC and reopen PowerVCR and the sound should be back. Not an ideal solution but at least you don't need to reboot.
As for no sound in the captured video, I have not yet found what the problem is there.