That's the thing about private drivers. Greece requires guides inside sites & museums to be highly trained & licensed ... many "car tours" present themselves as "guides" but in fact are simply hired drivers. The best-known guide in Delphi is Georgia Hasioti - https://sites.google.com/view/delphi-guide - she has rave reviews & is really tops. But of course it costs. The Delphi-guide site she has does not give a quote but her tour is 2.5 hours, and costs 172USD, for up to 10 people. This is a real bargain for a group but a splurge for a couple.
There are no audio tours I know of (R Steves' free audio tours in Athens are a welcome exception in Greece). Some other sites, such as Knossos in Crete, licensed guides sometimes wait at the site entry and collect a group, for €15-20 per person, but not Delphi. If you are the sort that likes to bone up beforehand, I have a list of excellent websites with maps & info you can print out.