Hmm, seems I missed this post.
If you want to extract files, there are some release packages for the
OP2Archive project.
If you're looking for example C++ code of how to do it yourself, the backend code can be found in the
OP2Utility project. Though if you're looking for historical code, there is the now unmaintained
OP2Editor (backend) project, which was an earlier source of example code.
You may also encounter an issue with the Sierra packed assets being in a non-standard custom format. Just extracting the files from the VOL might not be enough. At some point, someone repacked them in standard Windows bitmap format, which the game is able to read, though due to copyright reasons, I don't believe they are available for download anymore. I don't believe there is currently a utility that lets you easily convert, though the projects above do have some support for internal conversion of the two formats within their code. A proposal was suggested to create such a utility. Previously there wasn't much need, due to the repacking in standard format, though now that is no longer available, so a conversion utility may become a higher priority.