With a project open, you have to do a 'save as' to a completely different location, making sure you've got "copy audio with project" checked.
This will only copy across the audio being referenced by the project, meaning you can, if you want, delete the original project which might contain a lot of redundant audio clips.
You have to do this for every project, one at a time, and when you've finished, assuming you've deleted the originals, you still haven't archived anything - all you've done is tidied up your projects and probably reduced the hard drive usage by a fair amount.
This is a good time to take ANOTHER copy of everything from wherever you've done your "save as" to, maybe to an external drive.