Mpv cannot find fftw library

When I try to run mpv to watch a youtube video, e.g.:

mpv [url]

it gives the error:

mpv: error while loading shared libraries: libfftw3.so.3: cannot open shared object file: No such file or directory

I installed FFTW3 from source for my study some months previously, as I didn't know there was an apt package for it. Maybe that has something to do with it.

How do I fix this?

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