So, sound in flash was working for a short while and all of a sudden it stopped working. Other media players are (or may be) generating sound but the flash browser plug-in. I've experienced this and i have one possible solution that might work for you as it did me.
First, make sure you have the PulseAudio volume controller installed.
- sudo apt-get install pavucontrol
Second, make sure you're on a website that is using flash to play sound. Now open the PulseAudio volume controller and look for the flash plug-in. If it is muted, then unmute it. If the volume is all the way down, raise it up. If it isn't muted and not all the way down, then change the output device to a different one (this is what did the trick for me). If neither of these options bring back the sound, then you may have something deeper going on.