It happens that when I installed the adobe flash-plugin for Firefox, libflashsupport was not among adobe flash plugin’s dependencies.
Unfortunately without libflashsupport, there isn’t any sound on flash videos.
If you are encountering the same issue, yum install libflashsupport from adobe flash repository.
Then test it with this video (Le Train pour Pau):
Video recommended by Guillaume Assens.
Filed under: firefox