With the Nokia Suite software it should be quite possible to extract the contents of ARC backups and recover any audio files, including the Nokia Audio REL files.
Convert Nokia backup archive to Nokia audio file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert arc to rel on Windows
A set of applications used to manage older Nokia mobile phones
Convert from arc file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.