FIX
To get FLV functioning properly, try running this in your terminal:
sudo apt install ubuntu-restricted-extras
WHY THIS WORKS
The ubuntu-restricted-extras package contains audio codecs. Notice the word 'restricted.' This word is used by ubuntu because the package contains content that
THE ISSUE
First of all, according to your comments, ffmpeg is not installed. But I guess you know that much by now.
The issue here is that you replaced a codec; specifically your FLV codec. The ffmpeg FLV codec does work, it just probably is not compatible with the one that you were using before.
A codec has two main jobs: compression and decompression. To achieve these jobs, the programmer has to write an algorithm for each. The first has to give back a smaller version of the file. It is up to the programmer to decide how to do this.
This is important because a set of identical files that are compressed by different versions (implementations) of the "same" codec are different.
This is why your FLV files aren't working. Your new codec is trying to decompress files that it did not compress.
When a codec decompresses a file that it compressed, It should get back something really close to the original. (How close depends on the type of codec. For more info, research lossful and lossless compression formats.)
Note: When you switch out the decompression algorithm, the same thing should happen. You should get back something resembling the original. However, in the real world, standards aren't always followed.
In fact, in researching for this question, I learned that some codec formats are not even officially standardized. These are made up of just a basic idea and a free-for-all. I would guess that FLV is one of these. This is probably why it is not as popular as, for example, MP3.
