That sounds like what happens if you have a bad connection to the speakers or the amplifier in the speakers develops a problem.
If you use a headset, either to the front panel headphone jack or to the rear GREEN speaker jack, is the sound OK or do you still have the problem. If its OK then the problem is in the speaker system. If its still bad, power off the PC, reseat the SoundBlaster sound card (unplug from the expansion slot and then plug back in). Power back on and see if that helps.