Thanks for your help.
All the audio files are viewable in the sound effects browser, but none will play. The message at the bottom of the panel is "1234 Files Are Unplayable And Unavailable"
The media is stored in: Library->Audio->Apple Loops->Apple->Final Cut Pro Sound Effects
I've tried deleting FCP Preferences but it didn't fix it.
I've also downloaded Etrecheck and run it. It found no major problems, only minor ones (none of which seem obviously connected to FCP). Report uploaded as "Extra Text".
I have enabled Full Drive Access within Etrecheck and within System Preferences:Privacy so I'm not sure why it thinks I haven't!
I haven't made any major changes/installations since the last time FCP was working fine.
I'm using FCP version 10.4.10 on Catalina 10.15.7 (19H2) on a MacBookPro (2019 model). Ample disk space and RAM (64GB).
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