How to get apps detect phonon_backend

I installed Okular 1.10 via snap and, unlike the previous versions, it can't play videos embedded into pdfs due to the backend problem:

WARNING: bool Phonon::FactoryPrivate::createBackend() phonon backend plugin could not be loaded
WARNING: Phonon::createPath: Cannot connect Phonon::MediaObject ( no objectName ) to  Phonon::AudioOutput ( no
objectName ).

No matter how hard I try to export QT_PLUGIN_PATH with all the libs installed by

sudo apt install phonon4qt5 phonon4qt5-backend-gstreamer phonon4qt5-backend-vlc

or

sudo apt-get install phonon-backend-vlc

it still yields the same error.

strace doesn't even help to find out where Okular is trying to search the backend for.

Any hints?

I install Okular via snap because the Ubunutu's repository has an old version with a bug which nobody takes care of, namely, crash after quitting presentation mode.