I use KDE.
My web browsers recently quit playing sounds. If I download a file and try to play it, mplayer will play it happily, including flash videos downloaded from YouTube. However, the sounds & videos won't play from within the browser. Flash was installed, and has been re-installed per Leigh's instructions.
The only difference I can imagine that might be affecting it would be the installation of Skype. I have no idea how that might affect it, except that I removed Pulseaudio since Skype obsessed on it and ignored all other outputs. But PA wsas not my ordinary choice of output anyway, since it generally didn't work on the system. I had, under System Settings, already bumped up the hardware device (HD ATI SB ALC 1200 Analog).