if you have quality over ear hifi headphones they're likely to be better than the oculus ones.
Palmer Luckey describes himself as an audiophile, and he describes the built-in headphones as "well beyond decent". He also suggests that even if you have better quality headphones, the built-in ones will give you a better VR experience. This might be because positional audio works best if the sound engineer knows the exact specs and distances of the speaker drivers in relation to the ears.
The built-in headphones are pretty good, but it's hard to compare (there are so many different audio headsets of varying quality and cost). In any case, you can remove them and use your own if you wish.