I have an MKV file here that will play in the default Videos app but muted, and won't play at all in VLC (no error message, but a black screen and no sound). I already installed the recommended codecs for VLC, i.e. I ran the command
sudo apt-get install vlc-plugin-libde265.
When I open the file in Videos it says it can't find the plugin for "media files of the following type: audio/x-unknown decoder", then plays the muted video. Checking the file's properties does indeed show the audio codec as "unknown". I opened it in MediaInfo and it says:
English, 48.0 KHZ, 32 bits, 2 channels, Opus
So I tried to download the Opus codec, and Ubuntu told me it was already installed.