Convert THUMBDATA to MP3

Unfortunately, you cannot convert Android Gallery Thumbnail Index files directly to MP3 Audio Files (.thumbdata to .mp3 conversion). THUMBDATA files are found on SD cards of Android devices and contain cached thumbnail data of images. They neither contain documents, nor any other multimedia data, which is why you cannot extract, export or convert these files to any other format, including MP3.

Simply put, it is impossible to convert THUMBDATA to MP3 directly.
Thus, there is no so-called thumbdata to mp3 converter or a free online .thumbdata to .mp3 converter to be found.