Between 3.5 and 4.0 there was a rewrite of the audio backend. The audio is now synced to the game (like on a real GC or Wii), so if a CPU can't handle the game fullspeed, the audio will lag, stutter, or pop.
Switch over to the OpenAL audio backend and mess with the latency setting. This allows for time stretching of the audio thread so that it doesn't have as many problems.
OpenGL really should be giving you better performance than D3D. You should be running driver version 332.21 or 334.67 beta, as they give some OpenGL 4.4 speedups on the latest dev builds
Also, look into overclocking your CPU. More speed = more FPS
EDIT: ninja'd by Shonumi
Switch over to the OpenAL audio backend and mess with the latency setting. This allows for time stretching of the audio thread so that it doesn't have as many problems.
OpenGL really should be giving you better performance than D3D. You should be running driver version 332.21 or 334.67 beta, as they give some OpenGL 4.4 speedups on the latest dev builds
Also, look into overclocking your CPU. More speed = more FPS
EDIT: ninja'd by Shonumi