It would seem (from my limited understanding) that not having the version of libffmpeg.so that matches your Vivaldi install can be the cause of proprietary video/audio formats not functioning.
Here is the last post from this recent thread on this topic, found in the Vivaldi forum:
For anyone interested, here is a link to the start of the thread:Gwen-Dragon MODERATOR 25 days ago
Our Linux dev has been working on a fix to always use the correct libffmpeg version for Vivaldi.
I hope next Snapshot will give you a chance to test this.
So, it looks like those of us that use Vivaldi-stable should have a fix for this problem before too much longer.
[edit:] I just had a look at the snapshot 1.16.1170.3 release notes (30-April) & there is not mention of this bug, so hopefully we'll see something in the next snapshot?