Since a few days I have the problem that some videos do not output audio. This happens always in Twitter, not on Facebook and only with some videos of YouTube. The problem is present only in Firefox, in other browsers all videos play with audio. I have also tested with a clean profile with the same result. I have no problem with every other program I normally use. I guess that it is somehow related with the format of the audio, but I wasn't able to understand how to fix it, any hint?