Just to make sure : you did log out and then log back in after the modification, right? And you do have the "alsa-plugins" package installed?
Now I'd take a look if the flash wrapper's volume is controllable in the per-application part of the mixer. I'd also perform an "lsof /dev/snd*" while playing - sound working -a flash video (and while playing with vlc). If either VLC or flash are in the output, then they're not using pulseaudio. => the output should be something like
pulseaudio 18937 serafean mem CHR 116,5 2144 /dev/snd/pcmC0D0p