FCPX trial not allowing downloading additional content doesn't specifically mean it won't work with already installed sound files.
Since I have never used a trial version, I don't know if the following will work:
If your sound effects/music files (that aren't subject to DRM) are located in a folder on your system somewhere, make an alias of that folder. Then move the Alias into:
/Library/Audio/Apple Loops/Apple/Final Cut Pro Sound Effects [you may have to create this folder].
I have a ton of old Jam Pack sounds I bought years ago. They generally exist in
/Library/Audio/Apple Loops/Apple/* and I just made aliases of *these* folders (example: Jam Pack Remix Tools, Jam Pack Rhythm Section, etc.) and moved the aliases into the Final Cut Pro Sound Effects Folder making those older files available to FCPX without having to duplicate them.
Depending on the number of files you have available, be aware that it can take FCPX **several minutes** to actually begin to display them in the audio browser, so don't be put off if you don't see them right away.