There is a sound folder, named sound, not sounds. But that's the folder PaL meant, so there's nothing wrong.
Anyway, why should you want to unpack the .pak file? With Wally, you can browse through it, and so can Half-Life Model Viewer. It's easy to find the files you need, and they're useable just as good as if they were unpacked... You don't have to unpack it in order to use the files inside. Just browse through it with Wally or HLMV, and remember their file names and folders they're in, and fill that in in the entity properties...