I’ve found that loading a Flash applet in Firefox will cause Firefox to lock my whole system’s audio playback. Audio and video in Totem don’t even start to play when this happens. Until Firefox is closed, the only application that can play audio is Flash.
The libflashsupport package will fix this by forcing Flash to work properly with PulseAudio, which is new to Ubuntu 8.04. However, this package not installed by default because it has been know to cause stability problems.