I own a set of logitech usb headphones, which I occasionally like to use instead of the internal sound card in my laptop. When I go to the sound settings in gnome, and select "Logitech USB Headset" instead of "Built in Audio" all the system sounds, and sounds from audio and video apps go through the headphones except for flash videos which are running in a browser. It's the same in firefox and chromium. Does anyone have any idea about how to change this? I know it must be possible because it works by default in ubuntu.
I found the answer. I guess flash doesn't support pulseaudio out-of-the-box. But I installed libflashsupport-pulse in the aur and everything is working just fine. I hope this will help someone else who might run into the same problem.