It doesn't seem possible to convert Nokia audio (*.rel) directly to a Nokia SD card backup (*.arc). However, you could move the audio to the SD card and backup the whole SD card. But perhaps you were looking for the opposite ARC to REL conversion, that should be possible.
Convert Nokia audio file to Nokia backup archive.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert rel to arc on Windows
A set of applications used to manage older Nokia mobile phones
Convert from rel 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.