[Ubuntu 9.10] How to get rid of the emblem "disabled" of the Loudspeaker icon in »ALSA plugin [firefox]: ALSA Playback«?

[Detlef Lechner]

Hello,

[Ubuntu 9.10] GNOME
YouTube movie audio playback has stopped. Likely cause: I used other sound programs, for example Exaile and GNOME MPlayer.
Audio playback in Exaile works all right though. 
Clicking Applications > Sound and Video > 
PulseAudio Volume Control > (Volume Control) > Playback shows 
»System Sounds Mono 100%« but to the associated Loudspeaker icon is attached an "Disabled" emblem (a red circle crossed with a white bar)". The same is shown with »ALSA plugin 
[firefox]: ALSA Playback«. How to get rid of the two emblems in order to restore sound playback in YouTube movies?
http://linuxundich.de/de/ubuntu/sounds-mit-pulseaudio-umleiten/ did not enlighten me.

Kind regards,

Detlef Lechner