Ihesum is not standard Church Latin - in religious services, it would ordinarily be "Jesum" or "Iesum," since "J" and "I" are equivalent letters in Latin (there was no "J" in early Latin). I see many reference to "Ihesum" on the Internet, and they all seem to be very late medieval Latin. From what I can tell from recordings, all three spellings are pronounced the same way: YAY-soom.

-Joe-