You can unpack the archives using the "Keep broken files" option
from the "Extraction path and options dialog".

But most of the files don't work if they are broken.

You can try it. Perhaps a mpg video works, if you unpack
only the first parts.

For most of the files you need the file header.
The file header is placed in the first part of the file.
In the header you find the information, if it's a movie or
a text or a sound file.

So the best way is to download the first volume and extract it
with the "keep broken files" option.

If you have ie: "sample.r08", extraction only works, if
the multi volume archives are non-solid.
You can see information about the archive using the info function.

But not all movies will be played with the first part.
Some players check the length of the file and if it is different from
the header entry they report an error. The same occurs with pictures.