I set within EditVoicePack to ICAO (outside north america), but the ATC callout for QNH is in inHg, but it should be in hPa. Is this a bug or do I have to set something within P3D?
P3D4.5, EditVoicePack 6.2.0
This is indeed a change you need to make in the flight simulator itself.
EditVoicepack can change the words spoken by ATC, but only by replacing or adding. Changing the unit of massurement can only be done by performing performing a calculation, something that is not possible by modifying the data files only.