PC dosen't play sounds occasionally

My Pentium 3 (733Mhz) Desktop PC was running on Windows XP Professional SP2. The old (Creative Lab) Sound Blaster AWE 64 PCI sound card had been working fine. Recently I upgraded the OS to SP3. Since then, I only hear the audio part whenever I play a DVD video disc with Windows Media Player. But it's fine with VCDs. I installed K-Lite Codec Pack but that didn't solve the problem. So I have to play DVD videos with VLC Media Player. Today I played audio tracks on my PC for the first time ever since I upgraded the OS. Something strange happened. My PC stopped producing sounds after Windows Media Player or VLC Media Player played the 1st selected track on the playlist. I had to restart the PC to get the sound working again. But the same thing just repeated over and over and I couldn't find a solution. Please help.