I see. I know there are some relations between the language and the voice. It probably has something to do with voice recognition or speech rule.
Anyways. Narisa does not seem to be included by default in the US firmware. That may be the only problem. If you can email me a link to this voice, I'll add the required information in RGNVoiceEditor and we'll see if works.
turboccc at hotmail dot com
Edit: ok I re-read the thread and I know what is your problem. I should simply add Thai_Narisa in the program with the proper part number and it will work like the others. Stay tuned.
Edit2: Try version 1.08: [Only registered and activated users can see links. ]
Basically, create an rgn file with VPM_Tool. Run RGNVoiceEditor and change the desired voice. Save to RGN. If you want a GCD, just convert it back using RGN_Tool.