for playing system sounds
NS_SYSSOUND_* params are obsolete. The new events will not be supported by
this method. You should use playEventSound method instaed.
Not strictly necessary, but avoids delay before first sound.
The various methods on nsISound call Init() if they need to.
In some situations, playEventSound will be called. Then, each
implementations will play a system sound for the event if it’s necessary.
NOTE: Don’t change these values because they are used in
nsPIPromptService.idl. So, if they are changed, that makes big impact for
the embedders.