I recently acquired a new TV and hooked up an HDMI cable from my computer to it.
I spent quite a bit of time searching for a solution, found a lot of people have this problem on most computers, found a lot of complex solutions,
and then found the Windows 8 solution.
I suspect it applies back to XP also.
In Windows 8,
right click the sound icon in the tray,
select "Playback devices",
find and set your HDMI device to the default.
Every upgrade of Windows has done this to me.
Something that used to be difficult is now a right click away.