there are six Hebrew letters which have hard and soft forms: Beth-Veth, Gimel, Daleth, Kaph-Chaph, Pay-Fay, Tauv-Thauv.
the rules governing this are extremely technical. But here is a rough rule: If the word Pharoh BEGINS a new sentence or clause then it is pronounced hard (P vs F).
However if Pharoh is a middle or end word in a clause AND ALSO it follows an open syllable (like a word ending in Aleph Hey Vav Yud) then the Pay is pronounced softly (F vs P). The purpose of the soft pronunciation is to facilitate slurring the words of the phrase into one unit (to emphasize the clause as a unit).