If there is no check over the mentioned file you just need to replace the audio and png bytes inside the file taking care of maintaining the correct file size. Anyway I never modded it by myself because I was simply not interesd in doing that :P
Well, you need to know that the image (label) can not be related to the "isDemo" voice which is strictly (and correctly) related to TitleID, just a thing users should be aware of.
Necro-answer maybe useful for others: disc AES keys are different form NUS title keys (maybe they are also differently generated), this is the reason why they do not match.