sound problem

Hi,
I have faced a problem with sound. My volume control icon is not coming to the task bar. And voice playback default device has not selected.
I uninstalled my driver and reinstalled. but still sound is not come.
I can hear on windows media player. and also I can talk on yahoo messenger. but system sound is not come.before it was fine. My OS is XP with SP2.
now what should I do. Please hepl me.