How to change an alert sound in Karmic?

[Detlef Lechner]

Hi,

[Karmic] gnome-volume-control-applet > right-click Sound Preferences > 
(Sound Preferences) > 'Choose an alert sound:' is asserted to 
'Name=Default', 'Type=From theme'. How can I determine what file (name?) 
will be  played if an alert occurs?
If I am highlighted in Chatzilla, Ubuntu 9.10 will play the file 
/usr/share/sounds/ubuntu/stereo/bell.ogg. This sound is too short. How 
can I change this to playing the file 
/usr/share/sounds/ubuntu/stereo/phone-incoming-call.ogg?

Regards,
Detlef Lechner