It depends what is in them, doesn't it, Osipi? If these mysterious tapes contain just a rehash of stuff we have all heard before, a raking over the ashes of her marriage, then I doubt that people's opinion of Diana, whether favourable or unfavourable, would change at all.
If they contain advice to William and to his future wife about the pressures they will face as public figures, as has sometimes been suggested, then there might be a slight interest. However, as she was known as a devoted mother I don't think people's views on Diana would be changed at all, unless something inflammatory came out about Charles.
Personally I have great doubts about whether these 'secret' tapes exist at all.
What worries me about them, if they do exist, is that Diana apparently made them in 1997, at a time when she was at her most paranoid.
A former poster towards the beginning of this thread posted quotes from Diana's friends that she would write and phone them in the mid 1990's with comments like her being a star in the heavens looking down on them etc etc.
In other words if these tapes exist then they might go into her fears of being got rid of, assassinated while she was still young. If she was in that frame of mind then she might well have recorded allegations about Charles and/or members of his staff as a warning/guarantee that 'If anything happens to me, watch out! The scandals you've already gone through are nothing to this!'
Let me make my position clear. I do NOT believe that anyone in the royal family or people near to them were planning to kill Diana. That doesn't change the impression however that Diana appeared frightened and suspicious of people's motives at this time. Therefore she might, just might, have wanted to record what she knew, or thought she knew, about others.
If anyone knows about the actual physical recording of these tapes it would be Paul Burrell. I have become convinced since his curious rescue by the Queen before his trial that this person (grub though he is) knows more about the doings of individual members of the Royal family than he has ever let out in the public arena, in spite of his disgraceful books. If Burrell knows nothing of them, then IMO these secret tapes only exist in unnamed sources' fevered imaginations.