Ok, just rechecked these. I can change the sound file by either duplicating a character then changing the sounds in the FPE file, or changing them by the characters property panel, without errors. Both play side by side in the test game with the new assigned sounds.
Now I am on the private beta version at the moment, So hoping the next public preview release may be the cure to your issue.
Other then having you do a complete uninstall and reinstall, I don't have an answer for you, and I really don't want to have you go thru that at the moment. Let me check with another moderator who has multiple versions installed and see if they get the same results as you. Also I am not running win10, and they are.
Do you have the EBE stuff in your version of Game Guru right now?
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.