It sounds like the whole machine is slowing down, or something isn't writing to the sound buffer properly.
I believe that a number of people with c-media's ac97 have decreased the hardware acceleration and resolved this issue.
Run dxdiag, from start > run, goto the Sound tab (which has the c-media ac97 listed on it. It's probably Sound 1), and move the slider from Full Acceleration to a notch down (75%). That *may* help.