I tried mp4repair.org three years ago for a .3gp audio file on my Galaxy S4 recorded as AMR, and it worked in the end, after I had contacted them. I am however still looking for a way to do this for free.
You might want to ask Adithya in videohelp.com to help you. He fixed the file I had problems with for free. I offered to pay him, but he said there was no need, that I would pay him 5 bucks in the future if I had more files to fix. I haven’t needed his services again, but if I do, I will very gladly pay that, it’s more a token of appreciation than a real fee.
Yes, I have now asked Adithya whether he could explain how he fixes such corrupt files. I do have one file that is affected at the moment, but I would rather see a general guide, since I expect to have more such files in future. Besides, it would be educational, not only to me but also to many others.
I now also realise that Audacity would not be able to fix such files, as there is simply no usable sound code to work with.