Converting .thumbdata files directly to .mp3 audio format.
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.
In a word, direct thumbdata to mp3 conversion is not possible.
So don't expect any so-called thumbdata to mp3 converter or thumbdata to mp3 free online conversion tool available.
The database currently does not contain any thumbdata software converter.