Hello!
My name is Emanuele, I am a professional music producer and audio editor with a degree in Music.
I would be glad to help you with your issue! How many files are we talking about? Is the problem about the file itself, or when it is uploaded to the website?
Sometimes it can happen that the algorithm that the website/application uses varies the sample rate and resolution of the file and even if it sounds fine offline it is going to sound stretched/robotic on the platform.
In this case I suggest you to find a web developer that can help you, as much cannot be done on the file itself.
However, if are the files themselves the problem I can do something about it!
If they sound "robotic", like you say, it probably means that they had the original sample rate converted, or their resolution lowered without using a dithering. I could get my hands on those files and see if I can fix them by stretching and some proper use of equalisation and compression, but what I would rather do is having the "original" recording, or the files that were made with the original mic, because my impression after what you wrote is that the files have been messed with after their original recording and if it is possible to have those files it would be a lot easier to deal with the issue.
I have a lot of experience in editing and restoring audio for interviews, video games and film. I am sure we will be able to find a solution for your issue!
I hope to hear from you soon!
Cheers,
Emanuele