Conversion of undf files to mp3 file format

Undefined (UNDF) file types are typically partially downloaded, corrupted, or occasionally labeled as unknown by VLC player. Unless the file is actually just renamed audio file, there is no way how to convert or recover that to MP3. If you think the file is without any doubt audio, try to rename it to MP3 and play, if that doesn't work then there is really nothing you can do.

Open UNDF file    Open MP3 file