I think Oculus left the headphones apart on purpose.(Removed them and replaced them for build-in sound)
They could earn LOTS of money by selling those headphones apart from the Rift S.
Just think about it....
I mean, come on! Build-in 3.5mm port in the Oculus Rift S... No headphones on the headstrap, but there's plenty of space to place some headphones?
This mean it will be a nice accesiores they'll sell :-).