1) Restore the original voice in the OT or use a new one.
2) Keep Morrisson's voice the same for all his scenes that he doesn't wear a helmet in the PT.
3) Change Jango Fett's voice in the PT when he is wearing his helmet to imply a voice scrambler device.
This would mean we know what Jango Fett sounds like, we know his helmet has a scrambler. We never see Boba's face in the OT so whether his voice should be the same as his 'father' is no longer an issue.