After I installed Amarok (which I have uninstalled since then), Firefox doesn't produce sounds when I try to stream some websites, for instance these ones:
http://www.npo.nl/nos-studio-sport-eredivisie/09-04-2016/POW_03079959
http://www.dumpert.nl/mediabase/6750553/2f16a7a8/spartafan_heeft_een_favoriete_speler.html
But youtube does work on Firefox, and Chrome works perfectly fine with all websites.
I've tried to Google it and there are many suggestions on how to solve this problem (such as this one) but they don't solve my issue. Anybody knows how to solve this problem?
I've re-installed Firefox and followed all the tips (here)[https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues#w_plugins] but still no luck. I'm getting desperate.

sudo apt-get remove --purge firefoxthen remove.mozillafolder that is located in your Home Folder just press Ctrl+H to see it. Thensudo apt-get install firefoxafter you can sync it again with your account. – JoKeR Apr 14 '16 at 00:05