Removing and reinstalling all of the ATI software has restored sound.
Again, no hardware changes so it is a software setting. Now the only
problem is that the ATI software becomes "unresponsive" from time to
time; other applications continue to run without problems.
I'm having the same type of problem. A clean install of Windows XP,
installed Asus A8N-E chipset drivers and Realtek audio drivers, and
then ATI stuff. Started up TV, plays for 1 minute, and then freezes.
Before this, I had another clean install of XP. Sometimes it would
play for 10 minutes at a time, and one time it even recovered from a
freeze. I thought maybe it was because I had set all the settings to
high (my Athlon 64 dual-core should be able to handle it), but this
last time I just left everything at default. Now I've started checking
around on newsgroups, and it looks like lots of people are having
problems with the X600. I had kinda known that ATI's software was bad,
and had planned on using Windows Media Center Edition 2004 from a MSDN
disk, but then I found out that the drivers don't work with the card
unless you do some kind of hack/workaround. I put a half-hearted
effort into figuring that out, but only messed things up worse. That
is why I came to the fresh XP Professional (non-MCE).
Next step is to get SP2 and everything on, and then update Catalyst and
MMC from ATI website. I'm welcoming any advice.