no sound in firefox

Hello:
I am using 11.3 X64 and I found that there is no sound in my firefox flash video in KDE4, but it works well under gnome. Does anybody have any idea how to solve this? The sound for mplayer are OK for both gnome and KDE4.

THX:P

Check your PCM settings in your mixer as that can be inappropriately set to mute/0.