Most encoders seems to have a problem with DFF format (Direct Stream Digital Interchange Format), so your options for conversion of dff to mp3 audio are somewhat limited. One of the tools that supposedly supports conversion of DFF format is Audacity.
Convert Direct Stream Digital Interchange File Format file to Compressed audio and music file, mp3 songs or ringtones.
We found 3 software records in the database related to .dff to .mp3 file format conversion.
Microsoft Windows software - convert dff to mp3 on Windows
A free, open source software for recording and editing sounds
Convert from dff file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned dff to mp3. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert dff to mp3 on OS X
A multi-track audio editing tool
Linux/Unix software - convert dff to mp3 on Linux
Multi-track audio editor for Linux