No sound in mplayer

I have win98Se o/s and Via chipset Motherboard,my problem is,i installed win amp and mplayer9 ,when i access the winemp i can hear sound properly but in mplayer i can see the picture but no sound.some time it gives “can not play back the audio stream:no audio hardware is available,or the hardwre is not responding[error code=0x80040256 condition=15].Device driver,volume is proper.