after installing codec video can be open but no sound or image

Checking media player example on Linux Ubuntu:
Ubuntu 20.04.2 LTS

After installing codec:
anna:~$ apt list gstreamer1.0-libav
Listing... Done
gstreamer1.0-libav/focal,now 1.16.2-2 amd64 [installed]
gstreamer1.0-libav/focal 1.16.2-2 i386

After installation - the video can be open, but there is no video or sound.
Qt creator retuns warning:
18:53:01: Starting /home/anna/Qt/6.2/Examples/Qt-6.2.0/multimediawidgets/build-player-Desktop_Qt_6_2_0_GCC_64bit-Debug/player ...
platform is "xcb" 0x0
qt.multimedia.player: Warning: "No decoder available for type 'video/x-h264, variant=(string)itu, framerate=(fraction)25000/1001, width=(int)1280, height=(int)720'."

Additionaly - if additional libary is required - should it be added to the requirements?
https://doc.qt.io/archives/qt-6.0/linux-requirements.html

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Opening "avi" file of size 32 MB resulted in following error (belowy):

11:37:22: Starting /home/anna/Qt/6.2/Examples/Qt-6.2.0/multimediawidgets/build-player-Desktop_Qt_6_2_0_GCC_64bit-Debug/player ...
platform is "xcb" 0x0
qt.multimedia.player: Warning: "No decoder available for type 'video/x-h264, stream-format=(string)avc, alignment=(string)au, level=(string)5.1, profile=(string)high, codec_data=(buffer)01640033ffe1003327640033ad00ec03c0043ec05b808080a000007d20001d4c1c940000bebc20001312d3ef7064a00005f5e1000098969f7b804001000428ee3cb0, width=(int)3840, height=(int)2160, framerate=(fraction)30000/1001, pixel-aspect-ratio=(fraction)1/1, colorimetry=(string)bt709'."
qt.multimedia.player: Warning: "No decoder available for type 'audio/mpeg, mpegversion=(int)4, framed=(boolean)true, stream-format=(string)raw, level=(string)2, base-profile=(string)lc, profile=(string)lc, codec_data=(buffer)1190000000, rate=(int)48000, channels=(int)2'."
qt.multimedia.player: Warning: "No decoder available for type 'video/x-h264, variant=(string)itu, framerate=(fraction)25000/1001, width=(int)1280, height=(int)720'."