Using Vista Beta RC1 Catz 5 / Dogs 5
(install both and you get Petz 5)

The sound actually works without any trouble, unlike in XP.

Under XP with the latest DirectX 9.0c installed the sound nolonger works in Catz 5, Dogz 5, or Petz 5 (install both and you get Petz 5).

so either do NOT update Direct X because of one game or buy Windows Vista (which sound actually works with DirectX 9.0c.)

Unlike XP in windows vista there is some sort of problem where you can not access the .pet files for the game.
Example: if you adopt a pet from the adoption center or breed one into existence it doesn't appear in the adpted petz folder
-thus, unlike on Windows XP you are unable to adopt your petz out to other people in the community because even if you return the petz to the game they don't end up in the returned petz folder.
A patch needs to be made but the players of the petz community will likely have to create one on their own since UBIsoft never offered any real help for the petz game.